Make all struct variables optional

[JonathanMace]

All variables should be optional in a struct

[JonathanMace]

Also add 'one of' to structs

[JonathanMace]

To enable structs to have optional values, and still be lexicographically comparable, 'not present' is lexicographically less than 'any value'; the optional flag must therefore precede each variable as opposed to a single block of flags at the start of the struct.

This would be quite expensive as it entails an extra byte per field.

Ideally, we'd have really nice encoding that figures this out. But we don't. Do this at some point.