Allow direct access to the contents of MultiValue

[mpfaff]

Often the work I need to do on the contents of MultiValue does not require them to be in the order passed to the function. In these cases it would be more efficient to access the Vec directly without reversing its contents.

I suggest that a method be added to MultiValue called into_rev_vec(), which would return the Vec as-is. A matching from_rev_vec method should also be added to create a MultiValue from a Vec without reversing its contents.