PickN(slice, number) and Shuffle(slice) generators

[Bastes]

This adds 2 new generators:

• PickN(slice, number) generates a new slice of randomly-selected number (up to len(slice)) items from (slice) in the same order with no duplicates
• Shuffle(slice) generates a new slice with all items from slice in random order

Tests inspired by the existing generators'.

Neither generators have shrinker, as explained in their godoc, as the shrinking strategy is not obvious and the generators are intended to work on small slices.

Originally designed for go 1.22 and retrofitted to work with go 1.12

[Bastes]

(after a bit more thinking, I'm about to add tests for both generators that validate the same seed and input results in the same generation)

[Bastes]

(there, added tests for determinism + reworked both generators to use the built-in rng over re-using other generators)

[untoldwind]

Unluckly my current work related projects do not involve go, so it is hard to keep track of all the changes.
I updated the min version to 1.20.

[Bastes]

Well, you're quite reactive as far as I'm concerned ;) thank you.