Initially, invalidation support was limited to BaseKey, though it

was moved out early in development to InvalidatableMixin. The one

piece remaining in BaseKey was check_valid(), which raised a

KeyInvalidatedError if no longer valid.
This change renames the exception to InvalidatedError and moves

check_valid() out into the mixin, so that any invalidatable object can

perform these validity checks. It's been renamed to assert_valid(), to

make its purpose more clear.
When used as a context manager, invalidatable objects will now start

with a validity assertion, to avoid any surprises and catch problems

early.

Unit tests pass.