Refactor typcache.c's record typmod hash table.

Previously, tuple descriptors were stored in chains keyed by a fixed size
array of OIDs.  That meant there were effectively two levels of collision
chain -- one inside and one outside the hash table.  Instead, let dynahash.c
look after conflicts for us by supplying a proper hash and equal function
pair.

This is a nice cleanup on its own, but also simplifies followup
changes allowing blessed TupleDescs to be shared between backends
participating in parallel query.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm%3D34GVhOL%2BarUx56yx7OPk7%3DqpGsv3CpO54feqjAwQKm5g%40mail.gmail.com

src/backend/access/common/tupdesc.c

@@ -19,6 +19,7 @@
 
 #include "postgres.h"
 
+#include "access/hash.h"
 #include "access/htup_details.h"
 #include "catalog/pg_collation.h"
 #include "catalog/pg_type.h"
@@ -26,6 +27,7 @@
 #include "parser/parse_type.h"
 #include "utils/acl.h"
 #include "utils/builtins.h"
+#include "utils/hashutils.h"
 #include "utils/resowner_private.h"
 #include "utils/syscache.h"
 
@@ -443,6 +445,31 @@ equalTupleDescs(TupleDesc tupdesc1, TupleDesc tupdesc2)
 	return true;
 }
 
+/*
+ * hashTupleDesc
+ *		Compute a hash value for a tuple descriptor.
+ *
+ * If two tuple descriptors would be considered equal by equalTupleDescs()
+ * then their hash value will be equal according to this function.
+ *
+ * Note that currently contents of constraint are not hashed - it'd be a bit
+ * painful to do so, and conflicts just due to constraints are unlikely.
+ */
+uint32
+hashTupleDesc(TupleDesc desc)
+{
+	uint32		s;
+	int			i;
+
+	s = hash_combine(0, hash_uint32(desc->natts));
+	s = hash_combine(s, hash_uint32(desc->tdtypeid));
+	s = hash_combine(s, hash_uint32(desc->tdhasoid));
+	for (i = 0; i < desc->natts; ++i)
+		s = hash_combine(s, hash_uint32(TupleDescAttr(desc, i)->atttypid));
+
+	return s;
+}
+
 /*
  * TupleDescInitEntry
  *		This function initializes a single attribute structure in