Add -min-index and -min-index-by

[Fuco1]

This should return the index of the min/max element in the list.

Simple implementation is

(car (-grade-up '< '(3 4 1 0 2))) ;; => minimum is at index 3
(car (-grade-down '< '(3 4 1 0 2))) ;; => maximum is at intex 1

[emacksnotes]

R call these functions which.min and which.max. See https://www.rdocumentation.org/packages/base/versions/3.6.2/topics/which.min

See also Proposal add -which -- Return indices where the list entry is non-nil