Add scanLeft/scanLeftTail to all non-empty collections

[Prillan]

I was quite surprised to find that there were no scanLeft on any of the non-empty collections. Furthermore, the tail of a scan on a non-empty collection is itself non-empty, which might be a useful property to expose. The name scanLeftTail was the best I could come up with for now and it's of course open to bike-shedding.

Putting out this PR as a draft to get some input on it. Will add tests and documentation if you all consider it useful!

val nel = NonEmptyList.of(1, 2, 3)
val f: (Int, Int) => Int = ???
// Before
nel.toList.scanLeftNel(0)(f)
// After
nel.scanLeft(0)(f)

// And, before
nel.tail.scanLeftNel(f(0, nel.head))(f)
// After
nel.scanLeftTail(0)(f)

The others (NonEmptyChain, etc.) don't have the scanLeftNel syntax and requires more work to get the same result.

Finally, the implementations in this draft are a bit mixed. Don't worry about that for now. Again, I'm mostly checking if people would find this useful or not.