This adds RegisteredFieldProvider, which provides all the registered

fields and fieldsets in the same order used by the registries.
It also handles transformation of the new tracked bug fields. It will

transform a BugsField into the tracked fields during iteration. It

will also handle turning field ID lookups into tracked field instances,

whether or not they match a known tracker (by falling back to stored

information if needed), which is needed for change descriptions.

Unit tests pass.
Tested with the rest of the pending changes and verified all fields

were tracked.