chore(docs): Ensure roadmap is up-to-date (#615)

The roadmap contained some (very) oudated ideas regarding ideas for
future scope. This PR updates them with more recent text based on the
existing list of issues and implementation work that has happened over
the past few months.

---------

Co-authored-by: Benjamin Kietzman <[email protected]>

Author: paleolimbot

docs/source/roadmap.md

@@ -27,60 +27,63 @@ have not yet been scheduled for implementation.
 ## C library
 
 - **Type coverage**: The C library currently provides support for all types that
-  are available via the Arrow C Data interface. When the recently-added run-end
-  encoded (REE) types and potentially forthcoming string view/list view types
-  are available via the Arrow C Data interface, support should be added in
-  nanoarrow as well.
-- **Array append**: The `ArrowArrayAppend*()` family of functions provide a means
-  by which to incrementally build arrays; however, there is no built-in way to
-  append an `ArrowArrayView`, potentially more efficiently appending multiple
-  values at once. Among other things, this would provide a route to an
-  unoptimized filter/take implementation.
+  are available via the Arrow C Data interface except string view/list view types.
+  Support for these should be added in nanoarrow as well
+  ([#583](https://github.com/apache/arrow-nanoarrow/issues/583),
+  [#616](https://github.com/apache/arrow-nanoarrow/issues/616),
+  [#510](https://github.com/apache/arrow-nanoarrow/issues/510)).
 - **Remove Arrow C++ dependency for tests**: The C library and IPC extension rely
   on Arrow C++ for some test code that was written early in the library's development.
   These tests are valuable to ensure compatibility between nanoarrow and Arrow C++;
   however, including them in the default test suite complicates release verification
   for some users and prevents testing in environments where Arrow C++ does not
-  currently build (e.g., WASM, compilers without C++17 support).
+  currently build (e.g., WASM, compilers without C++17 support)
+  ([#619](https://github.com/apache/arrow-nanoarrow/issues/619)).
+- **Test verbosity**: Tests for the C library were written before testing utilities
+  in the `nanoarrow_testing` library were available (and before there was a
+  `nanoarrow_testing` library in which to put new ones). As a result, some of them
+  are very verbose and can be difficult to read, which can and should be improved
+  ([#577](https://github.com/apache/arrow-nanoarrow/issues/577),
+  [#566](https://github.com/apache/arrow-nanoarrow/issues/566)).
 - **C++ integration**: The existing C++ integration is intentionally minimal;
   however, there are likely improvements that could be made to better integrate
-  nanoarrow into existing C++ projects.
+  nanoarrow into existing C++ projects
+  ([#599](https://github.com/apache/arrow-nanoarrow/issues/599)).
 - **Documentation**: As the C library and its user base evolves, documentation
-  needs to be refined and expanded to support the current set of use cases.
-
-## IPC extension
-
-- **Write support**: The IPC extension currently provides support for reading
-  IPC streams but not writing them.
-- **Dictionary support**: The IPC extension does not currently support reading
-  dictionary messages an IPC stream.
-- **Compression**: The IPC extension does not currently support compressed streams.
-
-## Device extension
-
-This entire extension is currently experimental and awaiting use-cases that will
-drive future development.
+  needs to be refined and expanded to support the current set of use cases
+  ([#187](https://github.com/apache/arrow-nanoarrow/issues/187),
+  [#497](https://github.com/apache/arrow-nanoarrow/issues/497)).
+- **IPC Dictionary support**: The IPC extension does not currently support reading
+  dictionary messages an IPC stream
+  ([#622](https://github.com/apache/arrow-nanoarrow/issues/622)).
+- **IPC Compression support**: The IPC extension does not currently support
+  compressed streams using per-buffer compression, although streams can be compressed
+  outside the nanoarrow library (e.g., gzip compression of the entire stream)
+  ([#621](https://github.com/apache/arrow-nanoarrow/issues/621))
 
 ## R bindings
 
-- **Type support**: The R bindings currently do not provide support for extension
-  types and relies on Arrow C++ for some dictionary-encoded types.
+- **Conversion internals**: The initial implementation of conversion from
+  Arrow data to R vectors was implemented in C and its verbosity makes it
+  difficult to add support for new types. The internals should be refactored
+  to make the conversion code easier to understand for new developers
+  ([#392](https://github.com/apache/arrow-nanoarrow/pull/392)).
+- **Type support**: The R bindings currently rely on the Arrow R package for
+  conversion of some R types (e.g., list_of), and some types are not supported
+  in nanoarrow nor the arrow R package (e.g., run-end encoding, list view, and
+  string/binary view)
+  ([#617](https://github.com/apache/arrow-nanoarrow/issues/617)).
 - **ALTREP support**: A recent R release added enhanced ALTREP support such that
   types that convert to `list()` can defer materialization cost/allocation.
   Arrow sources that arrive in chunks (e.g., from a `Table` or `ChunkedArray`)
   currently can't be converted via any ALTREP mechanism and support could be
-  added.
-- **IPC support**: The IPC reader is not currently exposed in the R bindings.
+  added ([#219](https://github.com/apache/arrow-nanoarrow/issues/219)).
 
 ## Python bindings
 
-- **Packaging**: The Python bindings are currently unpublished (pypi or conda) and
-  are not included in release verification.
-- **Element conversion**: There is currently no mechanism to extract an element
-  of an `ArrowArrayView` as a Python object (e.g., an `int` or `str`).
-- **numpy/Pandas conversion**: The Python bindings currently expose the `ArrowArrayView`
-  but do not provide a means by which to convert to popular packages such as
-  numpy or Pandas.
-- **Creating arrays**: The Python bindings do not currently provide a means by
-  which to create an `ArrowArray` from buffers or incrementally.
-- **IPC support**: The IPC reader is not currently exposed in the Python bindings.
+- **Type support**: The Python bindings do not currently support unions,
+  string/binary view, or list view, or run-end-encoded types. When creating
+  Arrow arrays from iterables of Python objects, some types are not yet
+  supported (e.g., struct, list, datetime objects)
+  ([#618](https://github.com/apache/arrow-nanoarrow/issues/618),
+  [#620](https://github.com/apache/arrow-nanoarrow/issues/620)).