AstEnumerable bugs

[SeeminglyScience]

Using this issue to track bugs in the internal AstEnumerable class in the 0.5.0 preview build.

• [ ] for statements without an initializer throw NRE (e.g. for (; $i -lt $l; $i++) { })

• [ ] Any PowerShell class with a constructor will throw NRE (likely because ReturnType is null)