unordered_map,unordered_set: Fix excess list size for very low capacities

[stotko]

The size of the internal excess list of unordered_map and unordered_set is determined by the expected amount of collisions, but at least 1. However, insertion as a precaution requires a non-empty excess list to proceed even if the inserted element will not produce a collision. In case of very small capacities, e.g. n = 4, this may result in insertion failures if a collision happens. Increase the excess count by 1 to account for this "reserved" element.

Closes #411

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.33%. Comparing base (8f25f66) to head (baa4941).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #414   +/-   ##
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  Coverage   97.33%   97.33%           
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  Files          31       31           
  Lines        2512     2512           
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  Hits         2445     2445           
  Misses         67       67           

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