Add support for branch speculation

[lekcyjna123]

To use whole potential of out-of-order execution we need a speculation over jumps. As for now we stall whole pipeline till branch is retired. Instead of that we should:

• have a branch predictor
• check in fetch if fetched instruction is jump - if yes:
  • use branch predictor to get next_pc
  • make a checkpoint of current CPU state
• on mispredicted branch CPU state from before speculation should be restored and execution should be started from that point

Comments:

• first branch predictor can be very easy e.g. always current_pc+4
• if this will make a task easier we can assume speculation depth = 1