Bumps minimum required Rust version to 1.36

[jasongrlicky]

Note that this PR depends on https://github.com/chris-zen/coremidi/pull/18

Note: This is a breaking change.

All this PR does is bump the minimum-required Rust version in the README and rust-toolchain files, and fix any new warnings that appeared since Rust 1.30.

The motivation for this PR is to pave the way for:

• Allow the use of std::mem::MaybeUninit in fixing https://github.com/chris-zen/coremidi/issues/24 (requires Rust 1.36)
• Evenutally update outdated core-foundation and core-foundation-sys dependencies (requires Rust 1.36)
• Eventually update the crate to use the 2018 Edition of Rust (requires Rust 1.31)