feat(maitake): introduce a separate maitake-sync crate

[hawkw]

The maitake crate provides async synchronization primitives in a
maitake::sync module. These APIs don't require the rest of the
runtime, and would be nice to be able to use in other crates. maitake
is not yet published to crates.io, as it contains Git-only dependencies.
However, the sync module can be implemented entirely using published
code, and therefore, a crate containing only maitake::sync could be
published.

This branch introduces a new maitake-sync crate, containing the
synchronization primitives from maitake::sync. The maitake::sync
module now re-exports maitake-sync, so there should be no impact for
existing uses of the crate. The new maitake-sync crate can easily be
published to crates.io, and depended on from released code.