Pure ByteStream interface

[tomchristie]

This pull request ensures that our AsyncByteStream and SyncByteStream classes are pure interface classes, without any implementations themselves.

It also introduces the following concrete implementations, which act as helpful classes for developers, so that it's not always strictly necessary to implement a concrete class.

stream = httpcore.PlainByteStream(b"123")
stream = httpcore.IteratorByteStream(iterator=...)  # Optionally, also `close_func=...`.
stream = httpcore.AsyncIteratorByteStream(aiterator=...)  # Optionally, also `aclose_func=...`.

Developers don't need to use our concrete implementations, as they could instead provide their own implementations, but this change gives us both a nice purity of interface, and the convenience of providing a few very basic implementations.

[florimondmanca]

Some suggestions on naming, but 💯 sold on the idea!