This is the second attempt to fix our Git history. This should allow
"git blame" to work correctly in the upb/ directory even though our
automation unexpectedly blew away that directory.
- Add reference proxy type and change the scalar iterator to interact with it
- Add missing Iterator<T> -> Iterator<const T> conversion
- Add unit tests for reference and iterator types
- Add missing typedefs
- Add missing crbegin/crend functions
PiperOrigin-RevId: 544751376
After this change, `mini_table` only has MiniTable definitions themselves. Everything having to do with the MiniDescriptor wire format is in `mini_descriptor`.
Also rearranged some of the files in mini_table to have better structure for `internal/`.
This CL contains no functional change.
PiperOrigin-RevId: 543529112