Add associated functions

[kodek16]

Associated functions, or "static methods" in some languages, are functions belonging to a type namespace, but not requiring an instance of that type as a self parameter unlike methods.

Associated functions would be useful both for avoiding collisions in global namespace and potentially making traits more powerful by allowing a trait to specify required associated functions.

The proposed syntax is:

struct Foo {
  // A function defined within a struct definition without a self parameter should be
  // considered an associated function.
  fun bar() {
    // ...
  }
}

fun main() {
  // A new namespace navigation operator `::`.
  Foo::bar();
}