New slice helpers

[renom]

HasIndex([]T, i int) bool // check if the index exists.
At([]T, i int) T // return the value or zero value.
AtOr([]T, i int, fallback T) T // return the value or fallback value.

[samber]

HasIndex -> Find
At -> Nth
AtOr -> we can create a NthOr method

[renom]

May be NthOrEmpty too?