Add VariableListU8<N> and FixedVectorU8<N>

[paulhauner]

Adds special implementations of VariableList and FixedVector to optimise for the u8 type.

For reference: sigp/lighthouse#7952 (comment)

TODO

• See if @michaelsproul can get the compiler to optimise rather than add these impls.

[codecov]

Codecov Report

❌ Patch coverage is 51.21951% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.35%. Comparing base (3f3d7de) to head (5ab7494).

Files with missing lines	Patch %	Lines
src/variable_list_u8.rs	50.00%	31 Missing ⚠️
src/fixed_vector_u8.rs	52.45%	29 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
- Coverage   58.62%   58.35%   -0.27%     
==========================================
  Files           8       10       +2     
  Lines         290      413     +123     
==========================================
+ Hits          170      241      +71     
- Misses        120      172      +52     

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[michaelsproul]

I think we can close this in favour of my PR, if we are happy with introducing a small amount of unsafe code:

• Optimise SSZ encoding and decoding #55

[michaelsproul]

Performance-wise, the specialised type and the optimisation within the generic type are now equivalent (around 18us on my machine for 1M u8s). There were some kinks in the benchmarks that I had to iron out: with smaller lists and less warm-up time, the benchmark running first would run in 30us, and the one running second would run in 2us! The latest commit doesn't have this problem.