Centralize executor-related partitioning code.

Some code is moved from partition.c, which has grown very quickly lately;
splitting the executor parts out might help to keep it from getting
totally out of control.  Other code is moved from execMain.c.  All is
moved to a new file execPartition.c.  get_partition_for_tuple now has
a new interface that more clearly separates executor concerns from
generic concerns.

Amit Langote.  A slight comment tweak by me.

Discussion: http://postgr.es/m/1f0985f8-3b61-8bc4-4350-baa6d804cb6d@lab.ntt.co.jp

src/include/executor/execPartition.h

@@ -0,0 +1,65 @@
+/*--------------------------------------------------------------------
+ * execPartition.h
+ *		POSTGRES partitioning executor interface
+ *
+ * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/include/executor/execPartition.h
+ *--------------------------------------------------------------------
+ */
+
+#ifndef EXECPARTITION_H
+#define EXECPARTITION_H
+
+#include "catalog/partition.h"
+#include "nodes/execnodes.h"
+#include "nodes/parsenodes.h"
+#include "nodes/plannodes.h"
+
+/*-----------------------
+ * PartitionDispatch - information about one partitioned table in a partition
+ * hierarchy required to route a tuple to one of its partitions
+ *
+ *	reldesc		Relation descriptor of the table
+ *	key			Partition key information of the table
+ *	keystate	Execution state required for expressions in the partition key
+ *	partdesc	Partition descriptor of the table
+ *	tupslot		A standalone TupleTableSlot initialized with this table's tuple
+ *				descriptor
+ *	tupmap		TupleConversionMap to convert from the parent's rowtype to
+ *				this table's rowtype (when extracting the partition key of a
+ *				tuple just before routing it through this table)
+ *	indexes		Array with partdesc->nparts members (for details on what
+ *				individual members represent, see how they are set in
+ *				get_partition_dispatch_recurse())
+ *-----------------------
+ */
+typedef struct PartitionDispatchData
+{
+	Relation		reldesc;
+	PartitionKey 	key;
+	List		   *keystate;		/* list of ExprState */
+	PartitionDesc	partdesc;
+	TupleTableSlot *tupslot;
+	TupleConversionMap *tupmap;
+	int			   *indexes;
+} PartitionDispatchData;
+
+typedef struct PartitionDispatchData *PartitionDispatch;
+
+extern void ExecSetupPartitionTupleRouting(Relation rel,
+							   Index resultRTindex,
+							   EState *estate,
+							   PartitionDispatch **pd,
+							   ResultRelInfo ***partitions,
+							   TupleConversionMap ***tup_conv_maps,
+							   TupleTableSlot **partition_tuple_slot,
+							   int *num_parted, int *num_partitions);
+extern int ExecFindPartition(ResultRelInfo *resultRelInfo,
+				  PartitionDispatch *pd,
+				  TupleTableSlot *slot,
+				  EState *estate);
+
+#endif							/* EXECPARTITION_H */