Add RefinedTypeOps for companion objects of refined types

[fthomas]

This is a solution for #342 but unlike #346 it does not add a new macro. Compared to a macro solution, companion objects need be written manually and require a little bit boilerplate. E.g. for a refined type type PosInt = Int Refined Positive one would need to write a companion that extends from RefinedTypeOps like this:

object PosInt extends RefinedTypeOps[PosInt, Int]

Usage then looks like:

scala> PosInt(5)
res4: PosInt = 5

scala> PosInt(-5)
<console>:51: error: Predicate failed: (-5 > 0).
       PosInt(-5)
             ^

scala> PosInt.from(5)
res6: Either[String,PosInt] = Right(5)

scala> val PosInt(x) = 5
x: PosInt = 5

So this PR adds the RefinedTypeOps class to simplify the definition of companion objects of refined types, the RefinedType type class that combines RefType and Validate instances of a refined type (thus making #193 obsolete), and companion objects for refined types in the eu.timepit.refined.types package.

[codecov]

Codecov Report

Merging #369 into master will decrease coverage by 2.37%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #369      +/-   ##
==========================================
- Coverage   98.94%   96.57%   -2.38%     
==========================================
  Files          39       41       +2     
  Lines         476      497      +21     
  Branches        8       10       +2     
==========================================
+ Hits          471      480       +9     
- Misses          5       17      +12

Impacted Files	Coverage Δ
...ain/scala/eu/timepit/refined/api/RefinedType.scala	100% <100%> (ø)
.../scala/eu/timepit/refined/api/RefinedTypeOps.scala	100% <100%> (ø)

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[fthomas]

Another advantage of this approach over a macro that just adds some functions to an object is that we can easily add extension methods to any refined type companion. See #382 for details.