feat: range-over-func POC

[NathanBaulch]

I was surprised to see no mention of the upcoming range-over-func experimental feature in the issues/PR list. I haven't measured anything but in theory this feature could dramatically reduce slice allocations when composing "queries" over large collections since no intermediate slices need to be created.

Is there any interest in this? This POC PR demonstrates what iterator support might look like, including tests and examples. I went with an "I" suffix (to mirror the "F" convention) to differentiate the iterator functions from their slice.go counterparts.

For example:

iter := lo.ToIterator(1, 2, 3, 4)
result := lo.FilterI(iter, func(nbr int, index int) bool { return nbr%2 == 0 })
fmt.Printf("%v", result.Slice())

[NathanBaulch]

Probably worth thinking about if/how this would integrate with the new iter package in the stdlib.