Unifies JS & JVM implementation of JSR-310

[alonsodomin]

Removes all the JSR-310 backport stuff in org.threeten.bp by cross-compiling the java.time APIs in the JVM and JS, removing a lot of duplicate code.

[codecov-io]

Codecov Report

Merging #60 into master will increase coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   92.46%   92.63%   +0.17%     
==========================================
  Files          61       59       -2     
  Lines         955      923      -32     
  Branches       45       41       -4     
==========================================
- Hits          883      855      -28     
+ Misses         72       68       -4

Impacted Files	Coverage Δ
...d/src/main/scala/cron4s/lib/javatime/package.scala	85.71% <ø> (ø)
...ala/cron4s/lib/javatime/JavaTemporalInstance.scala	85.18% <ø> (ø)

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