Move code for managing PartitionDescs into a new file, partdesc.c

This is similar in spirit to the existing partbounds.c file in the
same directory, except that there's a lot less code in the new file
created by this commit.  Pending work in this area proposes to add a
bunch more code related to PartitionDescs, though, and this will give
us a good place to put it.

Discussion: http://postgr.es/m/CA+TgmoZUwPf_uanjF==gTGBMJrn8uCq52XYvAEorNkLrUdoawg@mail.gmail.com

src/backend/commands/indexcmds.c

@@ -24,7 +24,6 @@
 #include "catalog/catalog.h"
 #include "catalog/index.h"
 #include "catalog/indexing.h"
-#include "catalog/partition.h"
 #include "catalog/pg_am.h"
 #include "catalog/pg_constraint.h"
 #include "catalog/pg_inherits.h"
@@ -46,6 +45,7 @@
 #include "parser/parse_coerce.h"
 #include "parser/parse_func.h"
 #include "parser/parse_oper.h"
+#include "partitioning/partdesc.h"
 #include "rewrite/rewriteManip.h"
 #include "storage/lmgr.h"
 #include "storage/proc.h"

src/backend/commands/tablecmds.c

@@ -74,6 +74,7 @@
 #include "parser/parse_utilcmd.h"
 #include "parser/parser.h"
 #include "partitioning/partbounds.h"
+#include "partitioning/partdesc.h"
 #include "pgstat.h"
 #include "rewrite/rewriteDefine.h"
 #include "rewrite/rewriteHandler.h"

src/backend/commands/trigger.c

@@ -42,6 +42,7 @@
 #include "parser/parse_func.h"
 #include "parser/parse_relation.h"
 #include "parser/parsetree.h"
+#include "partitioning/partdesc.h"
 #include "pgstat.h"
 #include "rewrite/rewriteManip.h"
 #include "storage/bufmgr.h"