Get should validate the type of its input at runtime

[stuhood]

Currently, a Get does not validate the declared type of its input at runtime, which causes graphs to fail with exceptions in the wrong places.

While constructing a Get we should fail fast in the Get constructor. The effect of this will be a much more usable exception.

[stuhood]

The case where we were expecting a failure but didn't get it was related to #6657: the hypothesis was that @illicitonion was doing something like:

x = yield Get(X, Address, build_file_address)

... where build_file_address was of type BuildFileAddress, which is a subtype of Address, but not an exact match. We'd like the attempt to create an invalid Get instance like that to raise an exception.

[stuhood]

Fixed in #6731. Thanks @dotordogh !