Add 2-to-1 multiplexer
Add instruction fetcher. Stall not implemented yet.
Add 4-to-1 multiplexer
Modify the name of multiplexer testbench module: mux_tb -> MUX_TB for consistency
Add 32-bit value shifter
Add instruction decoder
Add todo list
Update todo list
Add instruction executor
Add data memory accessor
Add register file writer
Connect pipeline registers and pipeline blocks
Fix compile errors
FIX IREQ, DREQ value, line connection