Improve BRIN infra, minmax opclass and regression test

The minmax opclass was using the wrong support functions when
cross-datatypes queries were run.  Instead of trying to fix the
pg_amproc definitions (which apparently is not possible), use the
already correct pg_amop entries instead.  This requires jumping through
more hoops (read: extra syscache lookups) to obtain the underlying
functions to execute, but it is necessary for correctness.

Author: Emre Hasegeli, tweaked by Álvaro
Review: Andreas Karlsson

Also change BrinOpcInfo to record each stored type's typecache entry
instead of just the OID.  Turns out that the full type cache is
necessary in brin_deform_tuple: the original code used the indexed
type's byval and typlen properties to extract the stored tuple, which is
correct in Minmax; but in other implementations that want to store
something different, that's wrong.  The realization that this is a bug
comes from Emre also, but I did not use his patch.

I also adopted Emre's regression test code (with smallish changes),
which is more complete.

src/include/access/brin_internal.h

@@ -16,6 +16,7 @@
 #include "storage/bufpage.h"
 #include "storage/off.h"
 #include "utils/relcache.h"
+#include "utils/typcache.h"
 
 
 /*
@@ -32,13 +33,13 @@ typedef struct BrinOpcInfo
 	/* Opaque pointer for the opclass' private use */
 	void	   *oi_opaque;
 
-	/* Type IDs of the stored columns */
-	Oid			oi_typids[FLEXIBLE_ARRAY_MEMBER];
+	/* Type cache entries of the stored columns */
+	TypeCacheEntry *oi_typcache[FLEXIBLE_ARRAY_MEMBER];
 } BrinOpcInfo;
 
 /* the size of a BrinOpcInfo for the given number of columns */
 #define SizeofBrinOpcInfo(ncols) \
-	(offsetof(BrinOpcInfo, oi_typids) + sizeof(Oid) * ncols)
+	(offsetof(BrinOpcInfo, oi_typcache) + sizeof(TypeCacheEntry *) * ncols)
 
 typedef struct BrinDesc
 {