is-shallow-equal: Fall back to strict equality for non-object-like

[aduth]

This pull request seeks to resolve an issue / enhance behavior of isShallowEqual to account for cases where the arguments passed are not strictly object-like (arrays or objects). In particular, this is useful in cases where one of the two arguments may be undefined.

Implementation notes:

Included are some assorted general refactorings. After poring endlessly over benchmark results on trivial details, I've deemphasized "winning". The latest benchmarks still have the included implementation as the most performant option, though I have found these results to vary unexpectedly.

Testing instructions:

Ensure unit tests pass:

npm test

[codecov]

Codecov Report

Merging #116 into master will decrease coverage by 0.04%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
- Coverage   67.39%   67.34%   -0.05%     
==========================================
  Files          56       56              
  Lines         690      689       -1     
  Branches      142      143       +1     
==========================================
- Hits          465      464       -1     
  Misses        183      183              
  Partials       42       42

Impacted Files	Coverage Δ
packages/is-shallow-equal/index.js	100% <100%> (+3.03%)	⬆️
packages/is-shallow-equal/objects.js	100% <100%> (ø)	⬆️
packages/is-shallow-equal/arrays.js	88.88% <88.88%> (-11.12%)	⬇️

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