Add support for Enumerators

[jtoman]

A lot of our type signatures lose their expressiveness because we only support plain Enumerators. We should add support for Enumerators that are parameterized over a type. So Array#each called with no arguments should yield a Enumerator

This should be relatively straightforward, although figuring out how to do type inference on the contents of an Enumerator might be a little tricky.

[jtoman]

As an update, we've decided that enumerators are basically not inferenceable. That is, they can only be passed to positions where their type is required (such as foo: (Enumerator)) if they have been annotated.