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

src/tools/pgindent/typedefs.list

@@ -2208,6 +2208,8 @@ SSLExtensionInfoContext
 SSL_CTX
 STARTUPINFO
 STRLEN
+ST_ELEMENT_TYPE
+ST_POINTER_TYPE
 SV
 SampleScan
 SampleScanGetSampleSize_function