Replace `gen.Multi` and `gen.multi_future` with `gen.multi`.

Replace `gen.Multi` and `gen.multi_future` with `gen.multi`.

`multi_future` is awkward to type but is much more prominent
in native coroutines. The new function `multi` has a more
convenient name and delegates to either the YieldPoint
or Future version automatically.

For backwards compatibility, `multi_future` is still around,
and `Multi` was renamed to `MultiYieldPoint` so that we don't
have two different objects whose names differ only in case
(`Multi` is now an alias for `multi`).

See #1493.