Arbitrary instances for Vector/String with Size[P]

[fthomas]

This builds up on the work @matthedude did in #455 to provide Arbitrary instances for Vector and String with any Size[P] predicate.

The instance for String will become handy if we add something like FiniteString[N] (#437) to the library.

[codecov]

Codecov Report

Merging #471 into master will decrease coverage by 1.28%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #471      +/-   ##
==========================================
- Coverage   98.06%   96.78%   -1.29%     
==========================================
  Files          60       60              
  Lines         672      684      +12     
  Branches       14       12       -2     
==========================================
+ Hits          659      662       +3     
- Misses         13       22       +9

Impacted Files	Coverage Δ
...n/scala/eu/timepit/refined/scalacheck/string.scala	100% <100%> (ø)	⬆️
...ala/eu/timepit/refined/scalacheck/collection.scala	100% <100%> (ø)	⬆️

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[matthedude]

This is really cool great work 👍