Checkpoint:
  Fix array interface overlays always having a T of System.Object
Checkpoint:
  Extend JSIL.MakeArrayEnumerator and JSIL.GetEnumerator to take a second argument specifying the desired element type.
  Fix the array interface overlay so it produces enumerators with the correct element types.
  Kill dead JSIL.EnumerableArray type.
  Fix a generic argument typo.