This way, we do not have to tweak the `RegisterManager` to handle
multiple register types - we have one linear space instead. Additionally
we can use the bitset itself to separate the registers into overlapping
(the ones that are aliases of differing bitwidths) and nonoverlapping
classes (for example, AVX registers do not overlap general purpose
registers, thus they can be allocated simultaneously).

Another huge benefit of this simple approach is the fact that we can
still refer to *all* registers regardless of their class via enum
literals which makes the code so much more readable.

Finally, `RegisterLock` is universal across different register classes.