Add benchmarks for SSZ encoding/decoding

[michaelsproul]

These benchmarks can be used a baseline for measuring the performance improvements in:

• Optimise SSZ encoding and decoding #55

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.62%. Comparing base (10d63e4) to head (a5575b9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #58   +/-   ##
=======================================
  Coverage   58.62%   58.62%           
=======================================
  Files           8        8           
  Lines         290      290           
=======================================
  Hits          170      170           
  Misses        120      120           

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