Allow implicit parametric type specialization for `struct`s and `union`s

[Airtz]

Request for enhancement.

As of dev-2025-06 this works for procs :

proc($T/[]$U) {} //OK

Whereas you have to be explicit for structs and unions :

struct($T: typeid/[]$U) {} //Error: '$U' is not a type
struct($U: typeid, $T: typeid/[]U) {} //OK