Upgraded deps, fixed build, added code cov

[erikras]

General housekeeping. This code was getting dusty.

Moved tests to __tests__ dir.
Formatted code with Prettier
Added code coverage with nyc

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@652d6ae). Click here to learn what that means.
The diff coverage is 94.44%.

@@           Coverage Diff            @@
##             master     #81   +/-   ##
========================================
  Coverage          ?   96.2%           
========================================
  Files             ?       4           
  Lines             ?      79           
  Branches          ?       0           
========================================
  Hits              ?      76           
  Misses            ?       3           
  Partials          ?       0

Impacted Files	Coverage Δ
src/singletonCache.js	100% <100%> (ø)
src/deepEquals.js	100% <100%> (ø)
src/lruCache.js	92.85% <90%> (ø)
src/memoize.js	93.54% <90.47%> (ø)

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