- 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.