Rust requires all methods of impl Trait to be in a single file, which
means we had a giant backend/mod.rs covering all exposed methods. By
using separate service definitions for the separate areas, and updating
the code generation, we can split it into more manageable chunks -
this commit starts with the scheduling code.
In the long run, we'll probably want to split up the protobuf file into
multiple files as well.
Also dropped want_release_gil() from rsbridge, and the associated method
enum. While it allows us to skip the thread save/restore and mutex unlock/
lock, it looks to only be buying about 2.5% extra performance in the
best case (tested with timeit+format_timespan), and the majority of
the backend methods deal with I/O, and thus were already releasing the
GIL.
