Add more benchmarks

[soininen]

This PR adds a benchmark that was salvaged from a failed attempt to use numba JIT compiler to speed up MappedItem.__getitem__(). Some profiling experiments identified MappedItem.__getitem__() as a hot spot when using DatabaseMapping, so it could have been nice if it was possible to improve its performance.

No associated issue.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.25%. Comparing base (3c2b4b4) to head (9263b66).

Additional details and impacted files

@@           Coverage Diff            @@
##           0.8-dev     #389   +/-   ##
========================================
  Coverage    78.25%   78.25%           
========================================
  Files           75       75           
  Lines         9511     9511           
  Branches      2014     2014           
========================================
  Hits          7443     7443           
  Misses        1735     1735           
  Partials       333      333           

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