Support explicit return types in ProcLiterals

[HertzDevil]

Resolves #11218.

A ProcLiteral with an explicit return type can now be used to type instance and class variables:

class Foo
  getter foo = ->(x : Int32) : Int32 {
    return x // 2 if x.even? # okay
    x * 3 + 1
  }

  # current way:
  getter bar = Proc(Int32, Int32).new { |x|
    next x // 2 if x.even?
    x * 3 + 1
  }
end

The corresponding macro method accessor is ProcLiteral#return_type.