Use vendoring as the distribution method

[ZeroIntensity]

This eliminates function tables entirely, and instead ships the entire PyAwaitable source code as a header file. This may seem bad at first, but has a lot of nice benefits for us:

• PyAwaitable is no longer needed as a runtime dependency, as it will be completely baked into an extension.
• In turn, there's no chance of runtime collisions between installed versions of PyAwaitable.
• Thus, it's completely safe to pin a PyAwaitable version to an exact number without worrying about compatibility.

On the other hand, there a few downsides:

• Our ctypes bindings need to be written in favor of a C extension.
• There is some added complexity with how things are initialized, because a virtual extension has to be created and stored on the interpreter state.
• We're now completely reliant on a vendoring script, which could prove to be maintenance heavy. (But I doubt it will be worse than capsules.)
• Debugging will be screwed up a little bit downstream, because function names are mangled.

Closes #30 and #47.

Stale