Remove the fancy RegisterSet class introduced in 4c2bff61e.

While it was some nice and fancy template usage, it ultimately had many practical issues regarding length of involved expressions under regular usage as well as common code completion tools not being able to handle the structures.
Instead, we now use a more conventional approach which is a lot more clean to use.
Perhaps a maco or inline function? This is more self-documenting, even if it's borderline unnecessary.
I don't feel strongly about this, but unless its documented somewhere within the source, it's not clear why this is the way it is.