This increases the number of LUTs slightly (by ~20), as the immediate
mux is now separate to the main control unit, but I think it's worth the
cost.
The imm output is also renamed to imm_value. This is consistent with
rs1_value and rs2_value, and avoids a collision with the new imm output,
which represents the type of immediate.
This commit:
* changes the type of all output variables to logic
* splits variable declaration and assignment
* declares variables before modules that use the variables
