Use LOOP instead of MAPCAR/MAPC for pushing/popping register offsets.
Disable some tests in test/perltestdata, but add them to test/simple.

These are tests that even Perl gets wrong.
Record SUBREGISTER-COUNT instead of a list of SUBREGISTERS.

The numbers of registers nested within a register must be contiguous
with each other and with that of the parent register, so there's no
need to compute a list of subregisters--we just need to know how many
are nested within, and we can compute the rest therefrom.