Theminmax function implementation

This pull request implements theminmax function, which is very similar to themin andmax functions; however, makes it much more efficient to compute both themin and themax values in a single pass. This is extremely useful for a lot of applications, for example it can be used inside thetextwrap.dedent (#131919 ) function inside Python, themin andmax functions are also very often used inside statical methods to compute the range of values.

The advantage of this system is that instead of doing 2 passes inside the dataset / iterable, it can be once.

The function would be defined as:

(min_val, max_val) = minmax([1,2,34,45,6])

This probably needs a PEP (I am just familiar enough with the process so I am putting Pull request first)