Impls for function pointers are patchy

[petrochenkov]

I'd expect function pointers (fn() with all combinations of safety and ABI) to implement roughly the same set of traits as raw pointers, namely:
PartialEq, Eq, Clone, PartialOrd, Ord, Hash, fmt::Pointer, Debug

Currently Clone is implemented for safe and unsafe function pointers, but all the ABIs except for "Rust" and "C" are ignored, i.e. a paradoxical situation takes place - extern "rare_abi" fn() is Copy, but not Clone (Edit: and it causes ICEs).
PartialEq is implemented only for safe function pointers with "C" ABI.
All the other traits are not implemented.

[petrochenkov]

(I'm going to implement all of these if there are no objections.)

[bluss]

See also #24000 -- fn() types are also further parameterized by their number of lifetime parameters unfortunately, so it's very hard to cover even all possibilities of regular rust ABI functions.

[petrochenkov]

Hm, interesting

impl<T> Clone for for<'a> fn(&'a T) {}

works for plain references and other "concrete" uses of lifetimes, but I don't see any way to cover something like this

impl<T> Clone for for<'a> fn(T) where T: 'a {} // made up syntax

generically.