Support generic function declarations

[MaxDesiatov]

Inference and type checking within functions such as

func max<T: Comparable>(_ x: T, _ y: T) -> T {
  return x > y ? x : y
}

can be implemented with a creation of a "fake" concrete type T, which has all members of Comparable protocol copied into it. Then inference of func max could run as if T is a type constructor of a concrete type, not a generic parameter.