Convert all `put_instr` to `instr`.
Add CMP, MUL, DIV, REM. Rename ImmData to Constant.
Rename operand types.
WIP towards declarations. Operands can be optionally constant or assignable.
Stop calling semantic analyzer. Do we even need a separate one?
Add MOVE[ACMQ], CMP[I], narrowing and widening casts.
Add beginnings of error handling.