Remove types from internal slots.
Get rid of BaseReadableStream per #81.

- Move strategy stuff into BaseReadableStream, then rename it ReadableStream.
- The auto-teeing stuff from the old ReadableStream disappears (for now?).
- Forgot to rename _buffer to _queue in previous commit.
Get rid of BaseWritableStream per #81.

- Move strategy stuff into BaseWritableStream, then rename it WritableStream.
- Some buffer vs. queue cleanups.
- Closes #81.