Reorganize type class instances

[fthomas]

This is the same as #392 but on top of current master.

[codecov]

Codecov Report

Merging #438 into master will decrease coverage by 1.33%.
The diff coverage is 95.34%.

@@            Coverage Diff             @@
##           master     #438      +/-   ##
==========================================
- Coverage   97.99%   96.66%   -1.34%     
==========================================
  Files          57       57              
  Lines         649      660      +11     
  Branches       14       12       -2     
==========================================
+ Hits          636      638       +2     
- Misses         13       22       +9

Impacted Files	Coverage Δ
...ed/src/main/scala/eu/timepit/refined/boolean.scala	100% <100%> (ø)	⬆️
...red/src/main/scala/eu/timepit/refined/string.scala	100% <100%> (ø)	⬆️
...hared/src/main/scala/eu/timepit/refined/char.scala	100% <100%> (ø)	⬆️
...ed/src/main/scala/eu/timepit/refined/numeric.scala	100% <100%> (ø)	⬆️
...src/main/scala/eu/timepit/refined/collection.scala	100% <100%> (ø)	⬆️
...ed/src/main/scala/eu/timepit/refined/generic.scala	42.85% <27.27%> (ø)	⬆️

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[fthomas]

With 3c546f6 many of the numeric inference tests failed with Scala 2.10. Let's see if they become green again after moving the boolean inference instances.

[fthomas]

I've decided to limit this PR only to Validate instances because I had trouble moving the Inference instances without breaking tests. It is also not clear where put some of the instances that involve more than one predicate. For example, should implicit def existsNonEmptyInference[P]: Exists[P] ==> NonEmpty be in the Forall, Empty, or Not companion?