Type checkers for Python are great, but one of the things they do is

they'll flag code as unreachable if there are branches which are

checking the type of arguments, even if that branch exists in order to

provide runtime type-checking of arguments for public APIs.
typing.assert_never is designed for this case, but annoyingly it

always raises an AssertionError, and does not allow customization of the

message.
This change adds a new method, typelets.runtime.raise_invalid_type,

which can be used in the same way as assert_never, but defaults to

raising a ValueError (and allows different exception types if

desired), as well as taking in a message to include in the exception.

This allows us to write APIs which perform runtime type checking on

arguments without having the type checkers flag said checking as

unreachable based on the type hints.

Used this in some other code.