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

[tilk]

@lekcyjna123 Could you write a proper description?

[lekcyjna123]

Description has been updated

[piotro888]

link (not closing) #557

[tilk]

Shouldn't this be closed already? If you believe not, please write the reason.