Close #979 by adding encoders and decoders for MonthDay, Year, and ZoneOffset

[plokhotnyuk]

...and improve throughput of parsing and serialization of YearMonth values, by using build-in parser and formatter

[codecov-io]

Codecov Report

Merging #980 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #980      +/-   ##
==========================================
+ Coverage    85.3%   85.39%   +0.08%     
==========================================
  Files          75       75              
  Lines        2362     2362              
  Branches      159      159              
==========================================
+ Hits         2015     2017       +2     
+ Misses        347      345       -2

Impacted Files	Coverage Δ
...rc/main/scala/io/circe/numbers/BiggerDecimal.scala	90% <0%> (-0.63%)	⬇️
.../core/shared/src/main/scala/io/circe/Decoder.scala	92.83% <0%> (-0.32%)	⬇️
...les/core/shared/src/main/scala/io/circe/Json.scala	79.48% <0%> (+0.64%)	⬆️
...re/shared/src/main/scala/io/circe/JsonNumber.scala	92.3% <0%> (+2.56%)	⬆️
...re/shared/src/main/scala/io/circe/KeyDecoder.scala	97.29% <0%> (+2.7%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7765cc4...10df9db. Read the comment docs.

[travisbrown]

Thanks @plokhotnyuk! Will get this merged shortly.

[plokhotnyuk]

Also, I fixed formatting of comments and squashed commits.

As W/A it would better to add more tests for Java Time API codecs with custom formatters.

[travisbrown]

As W/A it would better to add more tests for Java Time API codecs with custom formatters.

Agreed.