Finished 8/16bit selection

Finished implementing 8/16-bit width selection and got the bugs worked out. Now we have 8-bit and 16-bit immediate loads, and 8-bit loads don’t trash the other 8 bits of the destination register. Still need to get 8-bit high-byte load implemented.

Next step is probably to rethink the hold_last function in the microcode assembler, should probably make it more intelligent to only hold level sensitive signals and leave the edge-sensitive ones out.