ioreg: Allow outer doc comments on fields
ioreg::union: Use a reasonable span
ioreg::register: Fix span associated with struct
ioreg::getter: Associate correct span with Getter type
ioreg::setter: Associate correct span with Setter type
ioreg::parser: Give correct span to field types
ioreg::parser: Give correct span to top level group
ioreg: set-to-clear registers should still have getters
ioreg::parser: Simplify field parsing

Also adds a bit of needed strictness.
Merge pull request #156 from bgamari/ioreg-docs

ioreg: More documentation fixes