- Make Registrar zero-copy. While it was using borsh for serialization,
the array of voting mint configs couldn't be serialized without
unaligned references.
- Reorganize all zero_copy fields such that switching to repr(C) would
not make a difference. (just for safety in case that happens)
- Add static asserts on the sizes of all account structs, so any changes
are visible very early.
- Add padding to structs that didn't have it yet.