Add `gi_state`, `cr_state`, and `ag_state` attributes to generators,
coroutines, and async generators respectively. These attributes return the
current state as a string (e.g., `GEN_RUNNING`, `CORO_SUSPENDED`).

The `inspect.getgeneratorstate()`, `inspect.getcoroutinestate()`, and
`inspect.getasyncgenstate()` functions now return these attributes directly.

This is in preparation for making `gi_frame` thread-safe, which may involve
stop-the-world synchronization. The new state attributes avoid potential
performance cliffs in `inspect.getgeneratorstate()` and similar functions by
not requiring frame access.

Also removes unused `FRAME_COMPLETED` state and renumbers the frame state enum
to start at 0 instead of -1.