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).
This is a pure git rename in order to make sure that git can follow
history. The next commit will integrate these into mdbook-core.
Additional commits will refactor/move/remove items.
This moves Result and Error to mdbook-core with the anticipation of
using them in user crates. For now, the internal APIs will be using
anyhow directly, but the intent is to transition more of these to
mdbook-core where it makes sense.
This is intended as a shared, internal library that will be used by
other mdbook crates. The intention is that those crates will either
directly use, or reexport items from this crate.
Initially this includes MDBOOK_VERSION, which will get reexported from
the preprocessor and renderer crates.
