Implement Optional[T] as an alias for Union[T, None] #477

In preparation for strict checking of None values, Optional[T] should be an alias for Union[T, None] (for arbitrary T).

There are at least two valid ways to implement this:

1. Special case Optional[T] in the semantic analyzer.
2. Support generic type aliases of form Optional = Union[T, None], and use this to define Optional.

In the latter case, the fact that Optional takes a type argument would be inferred from the right hand side -- I assume that T is a free (unbound) type variable, so it is inferred as an argument of the left hand side. The second case is also complicated by the fact that the result of Union[...] should support indexing.

The first approach is okay as well, even though eventually we might want generic type aliases.

In either case, the typing module must be modified as well so that Optional can be used at runtime.