Replace TrimmedString regex with Trimmed predicate type

[ceedubs]

This also adds TrimmedString.trim and is a replacement for #487 based
on the discussion there.

While we are at it I threw in an Arbitrary instance for TrimedString.

[ceedubs]

Adding Trimmed bumped us over the limit here. 41 seemed a bit too arbitrary so I bumped this to 45. I will not attempt to defend this solution.

[fthomas]

This looks great, thanks @ceedubs!

The build failed on 2.10 and 2.11 because MiMa complained about the added Arbitrary instance. We can add this in a binary compatible way via a new StringInstancesBinCompat1 trait - similar how new Arbitrary instances were added in https://github.com/fthomas/refined/pull/486/files

[codecov]

Codecov Report

Merging #504 into master will increase coverage by 1.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
+ Coverage   93.15%   94.37%   +1.22%     
==========================================
  Files          61       62       +1     
  Lines         672      676       +4     
  Branches       10       14       +4     
==========================================
+ Hits          626      638      +12     
+ Misses         46       38       -8

Impacted Files	Coverage Δ
...c/main/scala/eu/timepit/refined/types/string.scala	100% <100%> (ø)	⬆️
...n/scala/eu/timepit/refined/scalacheck/string.scala	100% <100%> (ø)	⬆️
...red/src/main/scala/eu/timepit/refined/string.scala	100% <100%> (ø)	⬆️
...in/scala-2.12/eu/timepit/refined/api/Refined.scala	0% <0%> (ø)
...c/main/scala/eu/timepit/refined/cats/package.scala	100% <0%> (+20%)	⬆️
...main/scala/eu/timepit/refined/scalaz/package.scala	100% <0%> (+72.72%)	⬆️

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[ceedubs]

Thanks @fthomas. I've hopefully fixed the binary compatibility.

[fthomas]

Looks great, thanks again @ceedubs!