Refactor typing_utils.narrow

[odashi]

This pr introduces some small changes in typing_utils.narrow:

• Write a complete documentation.
• Use __name__ in the error string rather than the str representation.
• Change the argument order to narrow(type, obj) to keep consistency with typing.cast

[odashi]

Some tricky error occurred around cast, investigating the cause.

[odashi]

cast does not work with TypeVar.
python/mypy#6440

As the type inference can be performed correctly without cast (because of the isinstance narrowing), simply avoiding cast may be the right choice for now.

[neubig]

LGTM, thanks!