Pyawaitable

- Moved away from function pointer tables for loading PyAwaitable--everything is now vendored upon installation.
- Improved performance with compiler optimizations.
- `PyAwaitable_` prefixes are now required, and the old `pyawaitable_*` functions have been removed.
- The warning emitted when a PyAwaitable object is not awaited is now a `ResourceWarning` (was a `RuntimeWarning`).
- `PyAwaitable_AddAwait` now raises a `ValueError` if the passed object is `NULL` or self, and also now raises a `TypeError` if the passed object is not a coroutine.
- Added a simple CLI, primarily for getting the include directory from `meson-python` (`pyawaitable --include`).
- PyAwaitable objects now support garbage collection.
- **Breaking Change:** `PyAwaitable_Init` no longer takes a module object.
- **Breaking Change:** Renamed `awaitcallback` to `PyAwaitable_Callback`
- **Breaking Change:** Renamed `awaitcallback_err` to `PyAwaitable_Error`
- **Breaking Change:** Renamed `defercallback` to `PyAwaitable_Defer`
- **Breaking Change:** Removed the integer value APIs (`SaveIntValues`, `LoadIntValues`, `SetIntValue`, `GetIntValue`). They proved to be maintenance heavy, unintuitive, and most of all replaceable with the arbitrary values API (via `malloc`ing an integer and storing it).

- Significantly reduced awaitable object size by dynamically allocating it.
- Reduced memory footprint by removing preallocated awaitable objects.
- Objects returned by a PyAwaitable object's `__await__` are now garbage collected (_i.e._, they don't leak with rare circular references).
- Removed limit on number of stored callbacks or values.
- Switched some user-error messages to `RuntimeError` instead of `SystemError`.
- Added `PyAwaitable_DeferAwait` for executing code without a coroutine when the awaitable object is called by the event loop.

- Added support for `async with` via `pyawaitable_async_with`.

- Added getting and setting of value storage.

- Changed error message when attempting to await a non-awaitable object (_i.e._, it has no `__await__`).
- Fixed coroutine iterator reference leak.
- Fixed reference leak in error callbacks.
- Fixed early exit of `pyawaitable_unpack_arb` if a `NULL` value was saved.
- Added integer value saving and unpacking (`pyawaitable_save_int` and `pyawaitable_unpack_int`).
- Callbacks are now preallocated for better performance.
- Fixed reference leak in the coroutine `send()` method.

- Initial release.