Prefer `Option_` mappings for more types over transparent mappings

In general, the explicit `Option_` mappings are easier for
downstream bindings to match against, and clearer for users anyway.
Sadly, originally we were trying to avoid them where possible, so
some types default to transparent mappings. Here we swap the
default, at least for manually-mapped types, with a few explicit
exceptions (that probably should be changed as well).