WIP: faster paired loadstore

~3 fewer instructions per operation (2 generated by JIT, 1 in jitasmcommon).
Not implemented yet in JITIL (still takes the old path).
Currently disables write protection on JitAsmCommon, because I'm not sure what
to do there.

The basic idea: make a function pointer table that's updated each time the
GQRs are, letting us simplify the process of calling the correct paired
(de)quantization function.

This also saves us one register in psq_l/psq_st, so we don't have to reserve
RSCRATCH_EXTRA.