feat: add a backtrack combinator to Gen.

[codyroux]

Because we are in meta, we now need to add an inhabited instance to Gen before defining recursive functions which is possible with exceptions.

Backtracking is only useful when generators can fail, but this is quite useful in "guess and check" situations. There may be some redundancy with functionality provided later in Testable.

I've also made pick public since it can be quite useful.

[hargoniX]

Looks reasonable to me, I'll wait on Sebastian's remark regarding meta to merge.

One thing: What's the idea behind calling this combinator backtrack? It doesn't really feel like there is backtracking going on to me?

[codyroux]

The tryCatch at line 153 (would this be nicer as a try ... catch ...?) is doing the actual backtracking, trying a given generator, dropping it from the pool if it fails, and retrying with the remaining generators.

[codyroux]

I guess whether this is "actual" backtracking can be debated.

[codyroux]

Oops, @hargoniX I noticed that backtrack was only polymorphic at Type 0. Fixed now but you might want to take a second glance.