nth max and nth min

[arunsrinivasan]

Look for linear average (and worst case) performance.

[MichaelChirico]

@arunsrinivasan is this identical to #3804?

[jangorecki]

Not identical but could use common internal C fun.

[2005m]

Is like the Rfast::nth function that I mentioned in #3804 ?

[ben-schwen]

This is de-facto free once #5167 is merged by returning the root of the heap once instead of extracting the min/max until the heap is empty.