Json primitive map keys

[OndrejSpanel]

The code submitted supports Short, Int and Long in both reader and renderer.

Can you please check if the overall style is acceptable for you? I am unsure about a few things:

• I made the JSON reader to accept Chars anywhere an integral type (int, long, short) is expected. I am not sure if this is desired behaviour. If the strings as numbers should be accepted in map keys only, please advise how can I recognize I am parsing a key
• there is very little error handling. Once the decoder decodes a Long sucessfully, it will be silently truncated to Int or Short if such type is expected. If you want more error handling, I can add it

[codecov-io]

Codecov Report

Merging #319 (9e46b0a) into master (4f4ac9c) will decrease coverage by 0.03%. The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #319      +/-   ##
==========================================
- Coverage   74.02%   73.99%   -0.04%     
==========================================
  Files          63       64       +1     
  Lines        4893     4902       +9     
  Branches      575      588      +13     
==========================================
+ Hits         3622     3627       +5     
- Misses       1271     1275       +4     

Impacted Files	Coverage Δ
core/src/main/scala/io/bullet/borer/Reader.scala	70.34% <50.00%> (-0.21%)	:arrow_down:
...main/scala/io/bullet/borer/json/JsonRenderer.scala	75.43% <100.00%> (-0.59%)	:arrow_down:
.../io/bullet/borer/derivation/DerivationConfig.scala	100.00% <100.00%> (ø)

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[OndrejSpanel]

@sirthias Can you please take a look and tell me what you think about it?