Better solution to instruction re-issue, much faster, needs more work.

Store previous PC in upper bits of Controller_threads.
(removes need for subtractor to I_mem: a big crit path)
Delay storing new PCs by one cycle, along with write thread number.
(Avoids a JMP->PCM critical path)
Needs re-factoring into modules.
Needs Assembler adjusted to generate correct .pc memory values.
Needs more testing.
Moved instruction annulling into a module

Makes for a cleaner RTL diagram, and better documentation.
Closes #29
Move pc_reg definition before use.

Otherwise Modelsim complains.
Moved thread number generation to a module.

Will be useful for issue #26 later on.
Clean up Controller RTL diagram too.