This commit refactors the structure of the emulator so that it can be
used more easily as the engine for an interactive debugger for MASM
code.

* Emulator is now structured such that it can be run step by step, each
  step producing an event of some kind depending on what occurred during
  that step and what, if any, breakpoints are present.
* Added support for more emulator events
* Breakpoints previously implemented to handle stepping through code
  have been removed in favor of APIs on the emulator itself for that
  purpose.
* Support for multiple breakpoints at once was implemented
* Support for watchpoints (i.e. monitoring changes to memory) was
  implemented
* Additional debug information is now provided upon request

The idea for `CompTest` is to mix and match sources (Rust, Wasm, IR)
and expected compiler output (IR, MASM) to test the whole
pipeline(Rust->Wasm->IR->MASM) or any part of it (Rust->IR, Wasm->IR, IR->MASM, etc).

The compiled MASM code is executed on the emulator and VM and the results
are compared with the native Rust code. The inputs are generated via `proptest`.
The Rust source code consists of the target function under test and an
app binary that calls the target function. The app binary is compiled
to Wasm and then to IR. The target function is called directly in the
test from the linked crate.

…o `tests` directory

…results;

Wasm frontend tests are converted to use integration tests infrastructure.

…ram` to Wasm frontend

…ests

Fixes `i64.extend_u32` by adding casts to u32 and back to i64.

…rates, one for Wasm and one for calls in tests

… integration tests

…tion

…neBlocks)

when producing IR for the integration tests.

…alue

when looking for the value definition in the predecessors. #51

This removes the remnants of the previously removed `ValueData::Alias` variant.
It does not matter if we find the definition in the predecessors since we cannot
swap all sentinel value uses with it. So this fix always makes the sentinel value into a
block argument.