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.
