Remove no-longer-needed ALU enabled status bit.

This is implied by the opcode's MSB.
Start of a list of control states.

This isn't complete yet since there are some missing data paths for the
implementation of the jump/branch instructions. However, it's enough to
make me suspect that 4 bits is enough to uniquely represent a useful
state, so I plan to use a map from states to control words to implement
the control logic, which should simplify the later logic for determining
the next state for a given state/opcode/LSB-of-L.