This switches all public types to use non_exhaustive to make it easier
to make additions without a semver-breaking change.
Some of the ergonomics are hampered due to the lack of exhaustiveness
checking. Hopefully some day in the future,
non_exhaustive_omitted_patterns_lint or something like it will get
stabilized.
Closes https://github.com/rust-lang/mdBook/issues/1835
This updates everything for the move of utils to mdbook-core. There will
be followup commits that will be moving and refactoring these utils.
This simply moves them over unchanged (except visibility).
