Extend ':=' operator any type with a KeyEncoder instance

[tejinders]

In the cases where JSON keys have a richer type
than plain strings, it is handy to have the ':='
operator work seamlessly. As circe already has
'KeyEncoder' abstraction, the assignment operator
can work off the same abstraction. This is also
backwards compatible with the 'String' based
implementation.

[codecov-io]

Codecov Report

Merging #986 into master will increase coverage by 0.33%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #986      +/-   ##
==========================================
+ Coverage   85.18%   85.52%   +0.33%     
==========================================
  Files          75       75              
  Lines        2362     2362              
  Branches      159      159              
==========================================
+ Hits         2012     2020       +8     
+ Misses        350      342       -8

Impacted Files	Coverage Δ
...hared/src/main/scala/io/circe/syntax/package.scala	100% <100%> (ø)	⬆️
...re/shared/src/main/scala/io/circe/KeyDecoder.scala	97.29% <0%> (-2.71%)	⬇️
.../core/shared/src/main/scala/io/circe/Decoder.scala	93.14% <0%> (+0.62%)	⬆️
...les/core/shared/src/main/scala/io/circe/Json.scala	79.48% <0%> (+0.64%)	⬆️
...rc/main/scala/io/circe/numbers/BiggerDecimal.scala	90.62% <0%> (+1.25%)	⬆️
...re/shared/src/main/scala/io/circe/JsonNumber.scala	92.3% <0%> (+1.28%)	⬆️
...d/src/main/scala/io/circe/NonEmptySeqDecoder.scala	100% <0%> (+8.33%)	⬆️
...rc/main/scala/io/circe/shapes/SizedInstances.scala	92.85% <0%> (+14.28%)	⬆️

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[travisbrown]

@tejinders This is great, thanks (especially for the tests)!