Added `std.algorithm.searching.minIndex` to get the index of the minimum element of a range.

`std.algorithm.searching.minIndex` gets the index of the minimum element of a range,
according to a specified predicate. The default predicate is "a < b".

-------
import std.algorithm.searching : minIndex;
int[] a = [5, 4, 2, 1, 9, 10];
assert(a.minIndex == 3);

int[] a;
assert(a.minIndex == -1);
-------
