Pull changes from rustversion v1.0.23.

rustversion logic update

Updated version detection logic to mirror rustversion v1.0.23 (previously v1.0.17).

This includes the following changes:

• Support for the mirai compiler wrapper.
• Set RUSTVERSION=1 during rustc --version command (fixes rustversion#67)

Note that only changes to sources in the build/ subdirectory of rustversion
are actually applicable to rustversion-detect.
Even then, some changes are not needed here (ex. no need to workaround -Zfmt-debug=none).

Added

• Add #[must_use] annotations to most public methods.