Add support for parsing or defining union discriminator
We support union discriminators in GI API but we've no support on the parser side for that, nor we've an annotation to define it.
I suppose it's easy to guess it it when we've just an integer type in an union of structs or other types, but otherwise we'd need another annotation (I was thinking at something like (union discriminator=FIELD)
).
See also gjs!770, !348 (merged)
Edited by Marco Trevisan