Switch to beartype for generic type checking.

[coady]

The core of the dispatch logic is both simple and fast, but checking generic types is neither. It would be nice to offload type checking to a package like beartype, particularly if it becomes a standard. But there are some obstacles.

• only supports sequences, not iterables: [Feature Request] Deeply type-check set and frozenset containers beartype/beartype#167
• does not support callable with contravariant args: [DOOR Bug] beartype.door.TypeHint ignores (co|contra|in)variance concerns beartype/beartype#190
• is_subhint (issubclass) and is_bearable (isinstance) are slower than in the existing subtype implementation