pg: Add support for record/composite types

This is simply magical. \o/

Vendored in khashl.h. I wouldn't have used it if this were the only
place where I'd need a custom hash table, but it should come in handy
for other tasks as well, especially when I get to implementing an LRU
for prepared statement caching.

(Can all be done with Perl HV's, but they're less efficient and more
cumbersome for these tasks)

FU.xs

@@ -10,6 +10,7 @@
 #include "perl.h"
 #include "XSUB.h"
 
+#include "c/khashl.h"
 #include "c/common.c"
 #include "c/jsonfmt.c"
 #include "c/jsonparse.c"