Move ALU instruction decoding to ID stages

[laforest]

Although the simplicity of just passing the opcode to the ALU to use a case statement (and some careful opcode encoding choices) to select the operation has appeal, it means decoding as we compute, and for non-trivial ALUs, there's not enough time.

Instead, decode and pipeline the opcode into control signals during the two stages where we read/write A/B, so the 4 ALU stages can perform more complex work. Relates a bit to issue #31 in that it modularizes the datapath.