Reorganize partitioning code

There's been a massive addition of partitioning code in PostgreSQL 11,
with little oversight on its placement, resulting in a
catalog/partition.c with poorly defined boundaries and responsibilities.
This commit tries to set a couple of distinct modules to separate things
a little bit.  There are no code changes here, only code movement.

There are three new files:
  src/backend/utils/cache/partcache.c
  src/include/partitioning/partdefs.h
  src/include/utils/partcache.h

The previous arrangement of #including catalog/partition.h almost
everywhere is no more.

Authors: Amit Langote and Álvaro Herrera
Discussion: https://postgr.es/m/98e8d509-790a-128c-be7f-e48a5b2d8d97@lab.ntt.co.jp
	https://postgr.es/m/11aa0c50-316b-18bb-722d-c23814f39059@lab.ntt.co.jp
	https://postgr.es/m/143ed9a4-6038-76d4-9a55-502035815e68@lab.ntt.co.jp
	https://postgr.es/m/20180413193503.nynq7bnmgh6vs5vm@alvherre.pgsql

src/include/partitioning/partbounds.h

@@ -11,7 +11,11 @@
 #ifndef PARTBOUNDS_H
 #define PARTBOUNDS_H
 
-#include "catalog/partition.h"
+#include "fmgr.h"
+#include "nodes/parsenodes.h"
+#include "nodes/pg_list.h"
+#include "partitioning/partdefs.h"
+#include "utils/relcache.h"
 
 
 /*
@@ -101,7 +105,34 @@ typedef struct PartitionRangeBound
 } PartitionRangeBound;
 
 extern int	get_hash_partition_greatest_modulus(PartitionBoundInfo b);
-extern int partition_list_bsearch(FmgrInfo *partsupfunc, Oid *partcollation,
+extern uint64 compute_hash_value(int partnatts, FmgrInfo *partsupfunc,
+				   Datum *values, bool *isnull);
+extern List *get_qual_from_partbound(Relation rel, Relation parent,
+						PartitionBoundSpec *spec);
+extern bool partition_bounds_equal(int partnatts, int16 *parttyplen,
+					   bool *parttypbyval, PartitionBoundInfo b1,
+					   PartitionBoundInfo b2);
+extern PartitionBoundInfo partition_bounds_copy(PartitionBoundInfo src,
+					  PartitionKey key);
+extern void check_new_partition_bound(char *relname, Relation parent,
+						  PartitionBoundSpec *spec);
+extern void check_default_allows_bound(Relation parent, Relation defaultRel,
+						   PartitionBoundSpec *new_spec);
+
+extern PartitionRangeBound *make_one_range_bound(PartitionKey key, int index,
+					 List *datums, bool lower);
+extern int32 partition_hbound_cmp(int modulus1, int remainder1, int modulus2,
+					 int remainder2);
+extern int32 partition_rbound_cmp(int partnatts, FmgrInfo *partsupfunc,
+					 Oid *partcollation, Datum *datums1,
+					 PartitionRangeDatumKind *kind1, bool lower1,
+					 PartitionRangeBound *b2);
+extern int32 partition_rbound_datum_cmp(FmgrInfo *partsupfunc,
+						   Oid *partcollation,
+						   Datum *rb_datums, PartitionRangeDatumKind *rb_kind,
+						   Datum *tuple_datums, int n_tuple_datums);
+extern int partition_list_bsearch(FmgrInfo *partsupfunc,
+					   Oid *partcollation,
 					   PartitionBoundInfo boundinfo,
 					   Datum value, bool *is_equal);
 extern int partition_range_bsearch(int partnatts, FmgrInfo *partsupfunc,
@@ -114,11 +145,5 @@ extern int partition_range_datum_bsearch(FmgrInfo *partsupfunc,
 							  int nvalues, Datum *values, bool *is_equal);
 extern int partition_hash_bsearch(PartitionBoundInfo boundinfo,
 					   int modulus, int remainder);
-extern uint64 compute_hash_value(int partnatts, FmgrInfo *partsupfunc,
-				   Datum *values, bool *isnull);
-extern int32 partition_rbound_datum_cmp(FmgrInfo *partsupfunc,
-						   Oid *partcollation,
-						   Datum *rb_datums, PartitionRangeDatumKind *rb_kind,
-						   Datum *tuple_datums, int n_tuple_datums);
 
 #endif							/* PARTBOUNDS_H */

src/include/partitioning/partdefs.h

@@ -0,0 +1,24 @@
+/*-------------------------------------------------------------------------
+ *
+ * partdefs.h
+ *		Base definitions for partitioned table handling
+ *
+ * Copyright (c) 2007-2018, PostgreSQL Global Development Group
+ *
+ * src/include/partitioning/partdefs.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PARTDEFS_H
+#define PARTDEFS_H
+
+
+typedef struct PartitionBoundInfoData *PartitionBoundInfo;
+
+typedef struct PartitionKeyData *PartitionKey;
+
+typedef struct PartitionBoundSpec PartitionBoundSpec;
+
+typedef struct PartitionDescData *PartitionDesc;
+
+#endif							/* PARTDEFS_H */

src/include/partitioning/partprune.h

@@ -14,9 +14,10 @@
 #ifndef PARTPRUNE_H
 #define PARTPRUNE_H
 
-#include "catalog/partition.h"
+#include "nodes/execnodes.h"
 #include "nodes/relation.h"
 
+
 /*
  * PartitionPruneContext
  *