Reuse milhouse subtrees to shrink inactivity_scores in memory

[michaelsproul]

Use a new Milhouse method called intra_rebase to reuse identical subtrees in inactivity_scores. This reduces the memory used by each inactivity_scores update from ~70MB+ to ~4-5MB.

To update to milhouse's main branch I needed to steal Paul's changes from this PR, which I squashed into a single commit to make it easier to remove unwanted changes from unstable:

• Bump SSZ version for larger bitfield SmallVec #6915

I tested this change using an even hackier branch here, which includes the Milhouse mem-usage changes: https://github.com/michaelsproul/lighthouse/tree/milhouse-mem-size-holesky-rescue

Command for testing:

RUST_LOG=debug lcli skip-slots --pre-state-path state_3714912.ssz --slots 32 --network holesky

Results:

Inactivity score intra-rebase time: 47ms
Post-state balances size (total/diff): 84594016-84594016 B/79744688 B
Post-state validators size: 522924016-522924016 B/9416 B
Post-state inactivity_scores size: 84596096-84596096 B/4696384 B