Macro to derive a function fn variant_index_arity(&self) -> (u32, usize)
the pair (variant index, variant arity).
Only works on enumerations, of course.
We initially used the convert-case crate, but it converts names like โI32โ
to โi_32โ, while we want to get โi32โ. We thus reimplemented our own converter
(which removes one dependency at the same time).