Make value of the implicit class private

[implmnt]

Since circe has dropped support for scala 2.10, there is no reason to use public val's in AnyVal implicit classes.
This should fix potential collisions with user defined fields (key, wrappedEncodeable)

See a similar issue in cats

[codecov-io]

Codecov Report

Merging #1291 into master will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1291      +/-   ##
==========================================
- Coverage   85.91%   85.75%   -0.16%     
==========================================
  Files          74       74              
  Lines        2535     2535              
  Branches      108      108              
==========================================
- Hits         2178     2174       -4     
- Misses        357      361       +4

Impacted Files	Coverage Δ
...hared/src/main/scala/io/circe/syntax/package.scala	100% <ø> (ø)	⬆️
...d/src/main/scala/io/circe/NonEmptySeqDecoder.scala	91.66% <0%> (-8.34%)	⬇️
modules/rs/src/main/scala/io/circe/rs/JsonF.scala	94.11% <0%> (-5.89%)	⬇️
...c/main/scala/io/circe/tests/examples/package.scala	94.11% <0%> (-4.42%)	⬇️
.../core/shared/src/main/scala/io/circe/Decoder.scala	85.25% <0%> (-0.27%)	⬇️
...re/shared/src/main/scala/io/circe/JsonNumber.scala	93.97% <0%> (+1.2%)	⬆️
...les/core/shared/src/main/scala/io/circe/Json.scala	78.78% <0%> (+1.21%)	⬆️

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[travisbrown]

@implmnt Thanks for catching this! Will get it merged soon.