[tonykorycki]

Closes #10

draft: CI will fail until src/control_unit.sv is implemented

Adds tb for the control unit that checks :

• all 2048 {opcode, funct3, instr[3]} combinations against the control table 4 times each wiht the other instruction bits randomized
• 13 real instructions
• no x/z on on any output
• funct3 == instr[14:12] whenever branch_decoder_on is 1

Also: gen_control_table.py now also generates ctrl_diff() so failures name the fields that differ and make parsing easier

[tonykorycki]

superseded by #33