pg: Add prepared statement caching

The tests are not as thourough as I would like. There's many ways to
mess this up.

I was initially planning to drop the ref on the prepared statement
immediately after executing the query, so that the $st object can be
kept around for introspection without consuming excess resources.
Unfortunately, PQcopyResult does not copy over information about bind
parameters, so we need another way to keep that information alive. I
ended up going for the simple solution: keep the ref on the prepared
statement...

FU.xs

@@ -116,6 +116,12 @@ void cache(fupg_conn *x, ...)
   CODE:
     FUPG_STFLAGS;
 
+void cache_size(fupg_conn *c, unsigned int n)
+  CODE:
+    c->prep_max = n;
+    fupg_prepared_prune(c);
+    XSRETURN(1);
+
 void disconnect(fupg_conn *c)
   CODE:
     fupg_conn_disconnect(c);