Add sort_template.h for making sort functions.

Move our qsort implementation into a header that can be used to define
specialized functions for better performance and reduced duplication.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CA%2BhUKGJ2-eaDqAum5bxhpMNhvuJmRDZxB_Tow0n-gse%2BHG0Yig%40mail.gmail.com

Author: macdice