Add a hash_combine function for mixing hash values.

This hash function is derived from Boost's function of the same name.

Author: Andres Freund, Thomas Munro
Discussion: https://postgr.es/m/CAEepm%3D3rdgjfxW4cKvJ0OEmya2-34B0qHNG1xV0vK7TGPJGMUQ%40mail.gmail.com
Discussion: https://postgr.es/m/20170731210844.3cwrkmsmbbpt4rjc%40alap3.anarazel.de

src/include/utils/hashutils.h

+/*
+ * Utilities for working with hash values.
+ *
+ * Portions Copyright (c) 2017, PostgreSQL Global Development Group
+ */
+
+#ifndef HASHUTILS_H
+#define HASHUTILS_H
+
+/*
+ * Combine two hash values, resulting in another hash value, with decent bit
+ * mixing.
+ *
+ * Similar to boost's hash_combine().
+ */
+static inline uint32
+hash_combine(uint32 a, uint32 b)
+{
+	a ^= b + 0x9e3779b9 + (a << 6) + (a >> 2);
+	return a;
+}
+
+#endif							/* HASHUTILS_H */