Add an abi module with code that is probably useful to all ISAs when
implementing this function.
Add a unit() method to RegClassData which can be used to index the
register units in a class.
The List and Dict types are no longer implicitly available. They must be
imported from typing.
Type annotations must appear before the doc comment in a function. Also
fix type errors in these functions that weren't detected before.
Ensure that the set of register classes is closed under intersection.
Provide a RegClass::intersect() method which finds the register class
representing the intersection of two classes.
Generate a bit-mask of subclasses for each register class to be used by
the intersect() method.
Ensure that register classes are sorted topologically. This is also used
by the intersect() method.
