Add min() and max() expression functions.

[Prof9]

This adds min() and max() expression functions which take any number (min 1) of int or float parameters. The result type is int if all parameters are int, otherwise the parameters are converted to float and the result type is also float.

One potential use case would be to get the maximum size of a number of files so you could allocate a buffer that is just large enough to hold any of those files.

[Kingcom]

Could you add a small test for these functions? Looks good otherwise.