Move inheritance expansion code into its own file

This commit moves expand_inherited_tables and underlings from
optimizer/prep/prepunionc.c to optimizer/utils/inherit.c.
Also, all of the AppendRelInfo-based expression manipulation routines
are moved to optimizer/utils/appendinfo.c.

No functional code changes.  One exception is the introduction of
make_append_rel_info, but that's still just moving around code.

Also, stop including <limits.h> in prepunion.c, which no longer needs
it since 3fc6e2d7f5.  I (Álvaro) noticed this because Amit was copying
that to inherit.c, which likewise doesn't need it.

Author: Amit Langote
Discussion: https://postgr.es/m/3be67028-a00a-502c-199a-da00eec8fb6e@lab.ntt.co.jp

src/include/optimizer/inherit.h

@@ -0,0 +1,22 @@
+/*-------------------------------------------------------------------------
+ *
+ * inherit.h
+ *	  prototypes for inherit.c.
+ *
+ *
+ * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/optimizer/inherit.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef INHERIT_H
+#define INHERIT_H
+
+#include "nodes/relation.h"
+
+
+extern void expand_inherited_tables(PlannerInfo *root);
+
+#endif							/* INHERIT_H */