Support negative indexes in split_part().
 
This provides a handy way to get, say, the last field of the string.
Use of a negative index in this way has precedent in the nearby
left() and right() functions.
 
The implementation scans the string twice when N < -1, but it seems
likely that N = -1 will be the huge majority of actual use cases,
so I'm not really excited about adding complexity to avoid that.
 
Nikhil Benesch, reviewed by Jacob Champion; cosmetic tweakage by me
 
Discussion: https://postgr.es/m/cbb7f861-6162-3a51-9823-97bc3aa0b638@gmail.com