Slice shuffle and unique pick generators

[Bastes]

Using this (awesome) library, I've written a slice shuffle generator again and again for some of my use cases.

I'm currently working on a merge request that integrates that to the gopter.Gen package along with tests.

It's using Fisher-Yates shuffle algorithm so it shouldn't be too long to run, and give a properly random result. I've considered a reducer but I'm not sure it's necessary, all my tests using this gave good enough feedback without it.

Let me know if there's something I haven't thought through, I'll be sending the MR whenever I've finished writing it.

[untoldwind]

I do not see a problem with that.
I guess this would be an alternative to the SliveOfN generator

[Bastes]

Well, you've got it:

#89

(took me a bit of time to retrofit to go 1.12 😉)

[Bastes]

(it also adds a PickN(slice, number) to pick random number items from slice in the same order without duplicates)

[Bastes]

(oops, forgot to close when merged ; thank you again 🙂)