Move definition of UNUSED from modified headers of libmpdec to
_decimal.c itself. This makes the vendored source closer to the
standalone library and fixes build with --with-system-libmpdec.

Tested to build fine with either system libmpdec or the vendored one.

…using recv_into instead of recv (#21442)

* bpo-41273: Proactor transport read loop to use recv_into

By using recv_into instead of recv we do not allocate a new buffer each
time _loop_reading calls recv.

This betters performance for any stream using proactor (basically any
asyncio stream on windows).

* bpo-41273: Double proactor read transport buffer size

By doubling the read buffer size we get better performance.

Keywords are present in the main module tab completion lists generated by rlcompleter, which is used by REPLs on *nix. Add all keywords to IDLE's main module name list except those already added from builtins (True, False, and None) . This list may also be used by Show Completions on the Edit menu, and its hot key.

Rewrite Completions doc.

Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>