Bump Bitfield smallvec to 128 to avoid heap allocations

[paulhauner]

Increases the size of the Bitfield on the stack to 128 bytes. If the smallvec exceeds this then it will allocate to the heap.

This has the effect of ensuring that the Attestation bitfield can live on the stack for all networks with up to ~2M validators.

Allocating the Attestation to the heap is slow and increase memory fragmentation. This affects attestation deserialisation (happens very frequently) and block deserialisation (happens less frequently, but is latency critical).

Why 80 bytes?

The number of validators supported by a given bitfield size is N * 32 slots * 64 committees * 8 bits.

At the time of writing, mainnet has ~1.1M active validators and Holesky has ~1.76M.

128 is a nice round multiple of 8 that fits Holesky without putting too much on the stack.

Notes

This PR originally appeared at sigp/ssz_types#35

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.50%. Comparing base (d1acd77) to head (b92a494).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #38   +/-   ##
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  Coverage   77.50%   77.50%           
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  Files          12       12           
  Lines         720      720           
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  Hits          558      558           
  Misses        162      162           

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[michaelsproul]

Nice