Add tests that work out shrink_to_fit and check_n_keys

[antifuchs]

This does get a few tasty percent of coverage back, but more so, also adds some useful behavioral checks.

Fixes #112

[codecov[bot]]

Codecov Report

Merging #113 (5c7d5c6) into master (3ba020f) will increase coverage by 1.25%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   96.24%   97.49%   +1.25%     
==========================================
  Files          30       30              
  Lines        2075     2156      +81     
==========================================
+ Hits         1997     2102     +105     
+ Misses         78       54      -24     

Impacted Files	Coverage Δ
governor/src/state/keyed.rs	100.00% <100.00%> (+30.90%)	:arrow_up:
governor/tests/keyed_dashmap.rs	100.00% <100.00%> (ø)
governor/tests/keyed_hashmap.rs	100.00% <100.00%> (ø)
governor/src/state/keyed/hashmap.rs	100.00% <0.00%> (+11.76%)	:arrow_up:
governor/src/state/keyed/dashmap.rs	100.00% <0.00%> (+13.04%)	:arrow_up:

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[antifuchs]

bors merge

[bors[bot]]

Build succeeded:

• all_tests
• cargo_clippy
• cargo_deny
• rustfmt