Add execute flushing (#137)

- Fix pipeline flush mechanisms by removing inflight instructions inside
functional units, recovering the rename map, and adding trace rewind
functionality
- Add FlushCriteria class for requesting flushes to and from the
FlushManager
- Update FlushManager to handle and arbitrate between multiple flush
sources
- Add flush request port from ExecutePipe to FlushManager intended to be
used for mispredicted branches
- Delay completing instructions in ExecutePipe by a cycle to fix execute
flush race conditions
- Add branch identification methods for Inst
- Add test to send random flushes from the branch unit

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Signed-off-by: danbone <daniel.d.bone@gmail.com>
Co-authored-by: Daniel Bone <daniel.bone@imgtec.com>