The Great Renaming, Chapter 11: Add QCheck.noshrink

[jmid]

In trying to close up the renaming adventure, I've gone over the "generator returning" shrink combinators.

Here's a status:

Combinator name	QCheck.Gen	QCheck.arbitrary	QCheck2.Gen
add_shrink_invariant	-	('a -> bool) -> 'a arbitrary -> 'a arbitrary	('a -> bool) -> 'a t -> 'a t
set_shrink	-	'a Shrink.t -> 'a arbitrary -> 'a arbitrary	('a -> 'a Seq.t) -> 'a t -> 'a t
no_shrink	-	-	'a t -> 'a t

Observations:

• QCheck.Gen has no shrinking, so these make little sense there
• the set_shrink signature is slightly different, but so is classical shrinking in QCheck vs. integrated shrinking in QCheck2
• no_shrink could be easily added to QCheck.arbitrary for consistency

This PR thus does the latter as it is a small convenience function.
While wanting to write a test for it, I was pleased to notice that we already had one 🎉 so I updated that one instead.
The update also documents that no_shrink is easier to write than set_shrink Shrink.nil... 🙂