Added flow types and migrated to jest and rollup

[erikras]

Pretty major modernization...

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@a2313aa). Click here to learn what that means.
The diff coverage is 94.64%.

@@            Coverage Diff            @@
##             master      #91   +/-   ##
=========================================
  Coverage          ?   96.25%           
=========================================
  Files             ?        4           
  Lines             ?       80           
  Branches          ?       27           
=========================================
  Hits              ?       77           
  Misses            ?        3           
  Partials          ?        0

Impacted Files	Coverage Δ
src/singletonCache.js	100% <100%> (ø)
src/deepEquals.js	100% <100%> (ø)
src/memoize.js	92.59% <90.47%> (ø)
src/lruCache.js	94.73% <92.3%> (ø)

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