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 is a pure git rename in order to make sure that git can follow
history. The next commit will integrate these into mdbook-html.
Additional commits will refactor/move/remove items.
