Fixes #1300

Includes some changes to make the interface a little nicer and to fix some issues I noticed while I was looking at this code with fresh eyes:

* `table_name()` -> `table_path_name()`
* `is_table_id()` requires characters and table_paths to be length 1
* Vectorise `table_path_components()` instead of `table_path_name()`
* Make `table_path_components()` an S3 generic (as an escape hatch if needed)