Decoder#validate allowing building the message from the trigger HCursor

[pfcoperez]

Resolves #1019

This PR pretends to enable generating validate messages from the HCursor value which triggered the failure by satisfying the passed predicate.

It overloads the original validate method which now gets implemented by invoking this new implementation.

[codecov-io]

Codecov Report

Merging #1020 into master will decrease coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1020      +/-   ##
==========================================
- Coverage   85.37%   85.29%   -0.08%     
==========================================
  Files          75       75              
  Lines        2386     2387       +1     
  Branches      174      167       -7     
==========================================
- Hits         2037     2036       -1     
- Misses        349      351       +2

Impacted Files	Coverage Δ
.../core/shared/src/main/scala/io/circe/Decoder.scala	92.92% <100%> (-0.29%)	⬇️
...e/generic/extras/decoding/EnumerationDecoder.scala	88.88% <0%> (-11.12%)	⬇️
...scala/io/circe/shapes/LabelledHListInstances.scala	92.59% <0%> (-3.71%)	⬇️
...re/shared/src/main/scala/io/circe/KeyDecoder.scala	97.29% <0%> (-2.71%)	⬇️
...rc/main/scala/io/circe/numbers/BiggerDecimal.scala	90.36% <0%> (ø)	⬆️
...re/shared/src/main/scala/io/circe/JsonNumber.scala	92.4% <0%> (ø)	⬆️
...les/core/shared/src/main/scala/io/circe/Json.scala	79.61% <0%> (+0.63%)	⬆️
...d/src/main/scala/io/circe/NonEmptySeqDecoder.scala	100% <0%> (+8.33%)	⬆️

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[travisbrown]

This looks reasonable to me, thanks! We'll be sure it makes it into 0.11.

[pfcoperez]

@travisbrown As discussed in #1025 (comment) , I've updated this PR so the new validate method receives a HCursor => List[String] function to represent both the predicate and the errors generated by falsifying it.

New changes in the last commit: ca44c79

[travisbrown]

Thanks, looks good to me!