Added `std.algorithm.searching.maxIndex` to get the index of the maximum element of a range.

`std.algorithm.searching.maxIndex` gets the index of the maximum element of a range,
according to a specified predicate. The default predicate is "a > b".

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

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