to_stata uint16

Simple changes to io/stata.py to write unsigned integers to Stata files. Sorry about the poor commit messages--I'm still new to git and wasn't able to correct it cleanly.

(See issue #7365)
This is not the correct behavior for this function.  This function ensures that all datatypes after it is run have a trivial mapping to Stata data types.  It would be simplest to simply upcast `uint`s to the next largest `int` which is always safe and then the other changes in the commit are not needed.