Add missing QCheck2 float tests and fix QCheck2.Gen.exponential shrinker

[jmid]

As part of #367 I discovered that QCheck2 was missing a bunch of expect tests for float compared to QCheck.

This PR therefore adds those tests as mostly a copy-paste job.

Adding the tests however turned out to be a good idea, because it revealed that QCheck2.Gen.exponential (added in 0.23) and the newly added QCheck2.Gen.float_exp from #367 (making exponential an alias) would shrink to infinity - not something an end-user would expect of a generator with an exponential distribution...

The reason is that integrated shrinking Gen.maps the shrinking candidate 0. to infinity.
To fix it, I've therefore decided to just let float_exp's resulting generator shrink towards 0. rather than having to special-case 0. in the mapped function (or as a shrink invariant).

The PR documents the situation

• 3908e7c adds the missing tests
• cf776d0 updates and documents the unwanted expect test outputs
• 8399c30 moves QCheck2.ml definitions to prepare for the fix
• e559a96 contains the actual fix
• 6791cc1 updates the expect test outputs to reflect the fix