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

[adonovan]

Very nice! Thanks for contributing this feature, with docs and tests.

[adonovan]

Looks great, thanks again.

[adonovan]

This phrasing nicely finesses both the question of content and of ordering.