Add arbitraries for not empty collection/string which do not ever fail to generate value

[matwojcik]

I've observed, that generation of String :| Not[Empty] tends to fail to generate value from time to time (even up to 1.5% from my tests) <- because the generator used underneath from time to time selects 0 size for collection.

This has caused some flaky tests for us when we had many such values generated and combined - then the probability of failing increases to 1 - 0.985^n where n is number of values combined.

Scalacheck has built in function Gen.nonEmptyBuildableOf which can be used here. Not Empty case may be the most useful Length[X] case so it makes sense IMHO to optimise here. Similar thing is done in refined.