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 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.
