Support enum defaults

[Manishearth]

It should be possible to do:

enum SignDisplay {
     #[default]
     Auto,
     Always,
     OnlyNegative,   
}

such that null coeces to Auto. This would translate to a default constructor in C++, and would allow null/undefined in JS (so you can omit fields of options bags)

Ideally DiplomatOptional (#132) gets this too, or at least supports this as #[permit_default]

[Manishearth]

@sffc and I discussed this a bit and 99% of the time these things show up in struct fields where we should just support Option instead.

#246