Fixed bug regarding String instantiation

[EliasC]

@albertnetymk suggested a refactoring of how Strings are instantiated,
which leaves the work of turning string literals into constructor calls
completely to the desugarer, leaving the typechecker oblivious to the
existence of two different kind of strings (C-level strings and
Encore-level strings). In the process, a bug was discovered and
squished:

class T

class Main {
  def main() : void {
    new String("foo");
    new String(new T); -- Crashes the compiler
  }
}

[albertnetymk]

Would merge it in an hour if no objections.