This adds the `Book::chapters` iterator (and `for_each_chapter_mut`) to
iterate over non-draft chapters. This is a common pattern I keep
encountering, and I figure it might simplify things. It runs a little
risk that callers may not be properly handling every item type, but I
think it should be ok.
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.
