Change uint aliases to just be subclasses do not override init with no-op.

old (note that the ellipsis here is not for code showing purposes, it's python code):

class ValidatorIndex(uint64):
    def __init__(self, _x: uint64) -> None:
        ...

new:

class ValidatorIndex(uint64):
    pass

The subclassing works just fine, and does not have all the edge-cases to think about when overriding the __init__. Also enables (read: makes IDE type hint checker happy) us to do ValidatorIndex(123) instead of ValidatorIndex(uint64(123)).

Also experimented with the use of __metaclass__ or Elements.elem_type, ElementsType.elem_type in type hinting. Intellij + Mypy are happy (surprisingly), but Python is not. So we would have to shadow the bahavior of Generic to do that kind of type-hinting. That is for another time, construction of types and passing them is the most important now. The getter/setter/iteration is not even picked up as a problem by mypy, while it could be better (if Python allowed us to).

And fixed an old validator_registry reference, which was missed (genesis trigger testing is in progress I believe)