Add reusable routine for making arrays unique.

Introduce qunique() and qunique_arg(), which can be used after qsort()
and qsort_arg() respectively to remove duplicate values.  Use it where
appropriate.

Author: Thomas Munro
Reviewed-by: Tom Lane (in an earlier version)
Discussion: https://postgr.es/m/CAEepm%3D2vmFTNpAmwbGGD2WaryM6T3hSDVKQPfUwjdD_5XY6vAA%40mail.gmail.com

src/include/lib/qunique.h

@@ -0,0 +1,65 @@
+/*-------------------------------------------------------------------------
+ *
+ * qunique.h
+ *		inline array unique functions
+ * Portions Copyright (c) 2019, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		src/include/lib/qunique.h
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef QUNIQUE_H
+#define QUNIQUE_H
+
+/*
+ * Remove duplicates from a pre-sorted array, according to a user-supplied
+ * comparator.  Usually the array should have been sorted with qsort() using
+ * the same arguments.  Return the new size.
+ */
+static inline size_t
+qunique(void *array, size_t elements, size_t width,
+		int (*compare) (const void *, const void *))
+{
+	char	   *bytes = (char *) array;
+	size_t		i,
+				j;
+
+	if (elements <= 1)
+		return elements;
+
+	for (i = 1, j = 0; i < elements; ++i)
+	{
+		if (compare(bytes + i * width, bytes + j * width) != 0)
+			memcpy(bytes + ++j * width, bytes + i * width, width);
+	}
+
+	return j + 1;
+}
+
+/*
+ * Like qunique(), but takes a comparator with an extra user data argument
+ * which is passed through, for compatibility with qsort_arg().
+ */
+static inline size_t
+qunique_arg(void *array, size_t elements, size_t width,
+			int (*compare) (const void *, const void *, void *),
+			void *arg)
+{
+	char	   *bytes = (char *) array;
+	size_t		i,
+				j;
+
+	if (elements <= 1)
+		return elements;
+
+	for (i = 1, j = 0; i < elements; ++i)
+	{
+		if (compare(bytes + i * width, bytes + j * width, arg) != 0)
+			memcpy(bytes + ++j * width, bytes + i * width, width);
+	}
+
+	return j + 1;
+}
+
+#endif							/* QUNIQUE_H */