The minmax function implementation

This pull request implements the minmax function, which is very similar to the min and max functions; however, makes it much more efficient to compute both the min and the max values in a single pass. This is extremely useful for a lot of applications, for example it can be used inside the textwrap.dedent (#131919 ) function inside Python, the min and max 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)