Fix join_low8 and join_high8
Support setreg and getreg for SI and DI
In get_modrm_reg(), support 8-bit registers and return a Register (not a ModrmValue)
Return the value of ModRM.reg from get_modrm()
Add instructions mov_modrm_byte, mov_mreg_modrm, mov_modrm_mreg, mov_reg_byte
Map opcodes
Panic on unrecognized opcode