Checkpointing part 1: CheckpointRAT, Scheduler integration

[piotro888]

Finally after two months all problems were (hopefully) solved and first part of checkpointing is here 🎉

This PR contains CheckpointRAT - main data structure that substitutes F-RAT: does renaming, instruction tagging, saving checkpointed FRAT state and rollbacks.

It integrates into Scheduler with two stages - tag (allocating tags, tagging instructions, stalling on rollback in progress) and rename (saves checkpoint and does rename for valid instructions).
Additionally Frontend is prepared for resuming from rollbacks (but no Fetch yet), and Retirement is only retired freeing tags for now.

No rollbacks are currently made, but CheckpointRAT is integrated with Scheduler in its final form, that is tested in checkpointing test on SchedulerTestCircuit, that simulates Frontend, FBs, Branch rollbacks, ROB and Retirement with targeted changes, and verifies register renaming and retiring of instructions only from correct speculation path.

TODO (draft):

• Clean up code
• Minor CRAT imporvements
• Submit Transactron PRs
• Some docstrings missing

For future PR:

• Integrate with Fetch
• Integrate with JumpBranch
• Support rollbacks in ExceptionCauseRegister
• Support in Retirement (interaction with exceptions)