Refactor instruction formats (make way for more Format A instructions)

[mbitsnbites]

The number of Format A instructions is relatively limited. We could make the following changes to the encodings:

• Format A+B: Bits 26-31 = 000000..000011, instead of just 000000 (i.e. quadruple the number of Format A & B instructions).
• Move Load/Store instructions to make room for the above change (there's still room in the 4-15 opcode range, both for Format A & C).

Furthermore:

• Consider removing some/all of the MUL/DIV/REM instructions from Format C to make room for more Format C instructions. The value of these instructions is very limited.