Adding additional set builtins

[SamWheating]

This PR enables the following operations:

• S.difference(x) AKA S - x
• S.intersection(x) AKA S & x
• S.symmetric_difference(x) AKA S ^ x
• S.issubset(x) AKA S <= x or s < x

• S.issuperset(x) AKA S >= x or S > x

  I also unified the unified the implementations of these operations which were inlined in the evaluator with the implementation defined in values.go, to reduce duplication.

  Demo:

  Welcome to Starlark (go.starlark.net)
  >>> x = set([1,2,3])
  >>> y = set([3,4,5])
  >>> x.intersection(y)
  set([3])
  >>> x.difference(y)
  set([1, 2])
  >>> x.symmetric_difference(y)
  set([1, 2, 4, 5])
  >>> x.issuperset(y)
  False
  >>> x.issuperset(set([1,2]))
  True
  >>> x.issubset(y)
  False
  >>> x.issubset(set([1,2,3,4]))
  True