v0.3.0

Note that clients using async primitives provided by Shuttle (task spawn, block_on, yield_now) will
need to be updated due to the renaming of the asynch module to future in this release.

• Rust 2021 conversion and dependency bumps (#76)
• Implement thread::park and thread::unpark (#77)
• Implement std::hint (#78)
• Rename the asynch module to future (#79)