Support fn() types as const generic parameters

[lyphyser]

Currently even with custom ADT const generics, fn() types are not allowed as const generic parameters types.

Adding that support would quite useful as it would allow to specify a ZST type that implements the Fn/FnOnce/FnMut traits by calling a const fn() value from a conat generic parameter (typically from an fb item), guaranteeing monomorphization.

Currently it seems the only way to do this is to manually create with a macro a separate type for each fn item.

[theemathas]

What's your use case for this? Using a F: FnOnce() generic already causes monomorphization, as long as this F type is a function item type, not a function pointer type.