Add a rust-toolchain file and document the required `cargo-xbuild` version

[jonlamb-gh]

The directory layout of rust-src has changed, using a nightly 2020-07-30 or newer results in this build error

error: rust-src component not found. Run rustup component add rust-src.

See also rust-osdev/cargo-xbuild#87

This will go away once we do #10