Check that generated ast respects type hints

[PhilReinhold]

We should try to always produce "valid" AST in accordance with the type hints defined on the AST dataclasses. We can check this mechanically in the tests.

There are two failures arising from the AST type hints themselves. A trivial failure comes from a failure in openpulse.ast to import typing.Union which causes type hint evaluation to fail. A less trivial failure is the fact that openqasm.ArrayDefinition doesn't allow DurationType for base_type. We should clarify if this is intended or not.

Edit: both failures are resolved in updated versions of the openpulse/openqasm dependencies.

[jcjaskula-aws]

Good to go. We need to:

• close #25
• open an issue to check element type in collections.