added BatArray.min_max

[UnixJunkie]

No description provided.

[gasche]

BatList.min_max takes a ?cmp:('a -> 'a -> int) parameter to specify an alternative comparison function (but .min and .max don't, weird). Do we want to do the same thing here? I think that the choice you made is also reasonable (and the code seems perfectly mergeable).

[UnixJunkie]

BatList.min_max is a much more recent addition than BatList.(min|max).
If we add a cmp optional parameter (defaulting to Pervasives.compare) that could be added
to all of them: min, max, min_max.

[UnixJunkie]

I think this should be accepted as is (because it is coherent with the rest of the Array module).
I we want to add cmp functions to BatArray.(min|max|min_max), please open an issue (I might do it, but later).