Drop parentheses around -> inside certain comma-separated lists

[HertzDevil]

Fixes #14216.

The formatter is not affected, since it will preserve any number of parentheses around a ProcNotation.

[HertzDevil]

I have been thinking about this: if macro interpolation transforms {T} to ::Tuple(T), {x: T} to ::NamedTuple(x: T), T[1] to ::StaticArray(T, 1), and T* to ::Pointer(T), wouldn't it make sense to simply turn all the ->s into ::Procs? Maybe that would be the simplest fix after all?

That leaves unions but IIRC there were still some edge cases making T | U and ::Union(T, U) not semantically equivalent (e.g. #11891).

[straight-shoota]

Yeah I think that makes sense 👍

[HertzDevil]

Well:

lib Lib
  fun foo(x : Int, Int)           # Error: expecting token '->', not ')'
  fun foo(x : Pointer(Void), Int) # Error: expecting token '->', not ')'
  fun foo(x : ::Proc(::Nil), Int) # Error: expecting token '->', not ')'
end

So ::Proc won't help here, but dropping the parentheses does, so let's go forward with this PR anyway. Also the return type of -> being optional makes things a bit complicated regarding #10931 (although -> cannot really be ambiguous as a block restriction).

[straight-shoota]

This seems awefully complex, but I guess there's no other way to handle the ambiguity issues around -> 😭