Expose more out/readfuncs support functions.

Previously bcac23d exposed a subset of support functions, namely the
ones Kaigai found useful. In
20160304193704.elq773pyg5fyl3mi@alap3.anarazel.de I mentioned that
there's some functions missing to use the facility in an external
project.

To avoid having to add functions piecemeal, add all the functions which
are used to define READ_* and WRITE_* macros; users of the extensible
node functionality are likely to need these. Additionally expose
outDatum(), which doesn't have it's own WRITE_ macro, as it needs
information from the embedding struct.

Discussion: 20160304193704.elq773pyg5fyl3mi@alap3.anarazel.de

src/backend/nodes/readfuncs.c

@@ -172,12 +172,6 @@
 	((length) == 0 ? NULL : debackslash(token, length))
 
 
-static Datum readDatum(bool typbyval);
-static bool *readBoolCols(int numCols);
-static int *readIntCols(int numCols);
-static Oid *readOidCols(int numCols);
-static AttrNumber *readAttrNumberCols(int numCols);
-
 /*
  * _readBitmapset
  */
@@ -2499,7 +2493,7 @@ parseNodeString(void)
  * Datum.  The string representation embeds length info, but not byValue,
  * so we must be told that.
  */
-static Datum
+Datum
 readDatum(bool typbyval)
 {
 	Size		length,
@@ -2556,7 +2550,7 @@ readDatum(bool typbyval)
 /*
  * readAttrNumberCols
  */
-static AttrNumber *
+AttrNumber *
 readAttrNumberCols(int numCols)
 {
 	int			tokenLength,
@@ -2580,7 +2574,7 @@ readAttrNumberCols(int numCols)
 /*
  * readOidCols
  */
-static Oid *
+Oid *
 readOidCols(int numCols)
 {
 	int			tokenLength,
@@ -2604,7 +2598,7 @@ readOidCols(int numCols)
 /*
  * readIntCols
  */
-static int *
+int *
 readIntCols(int numCols)
 {
 	int			tokenLength,
@@ -2628,7 +2622,7 @@ readIntCols(int numCols)
 /*
  * readBoolCols
  */
-static bool *
+bool *
 readBoolCols(int numCols)
 {
 	int			tokenLength,