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 removes the `pub` status of the SectionNumber field. The intent is
to make this potentially extensible in the future if we decide to add
more fields, or change its internal representation. With the existence
of the deref impls, generally this change shouldn't be visible except
for the constructor, which hopefully shouldn't be too cumbersome to use
`SectionNumber::new` instead.
This is a pure git rename in order to make sure that git can follow
history. The next commit will integrate these into mdbook-summary.
Additional commits will refactor/move/remove items.
