pg: Rework txn implementation + statement config API

I liked the Perl implementation of transactions, but managing state
between Perl and C is a bit cumbersome, so I've moved the whole thing
into C.

Also added a few statement configuration methods that currently don't do
anything yet.

c/pgtypes.c

@@ -3,7 +3,9 @@
 typedef void (*fupg_send_fn)(pTHX_ SV *, fustr *, void *);
 
 /* Receive function, takes a binary string and should return a Perl value.
- * libpq guarantees that the given buffer is aligned to MAXIMUM_ALIGNOF. */
+ * libpq guarantees that the given buffer is aligned to MAXIMUM_ALIGNOF.
+ * For fixed-length types, the recv function is only called after verifying
+ * that the input buffer has the correct length. */
 typedef SV *(*fupg_recv_fn)(pTHX_ const char *, int, void *);
 
 typedef struct {