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.

doc/src/sgml/brin.sgml

@@ -428,8 +428,8 @@ 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;
 </programlisting>
       <structname>BrinOpcInfo</>.<structfield>oi_opaque</> can be used by the