pgsql: Move some node-read support functions from nodes.h to readfuncs.

Move some node-read support functions from nodes.h to readfuncs.h

The functions moved in this commit have been exposed since c1ddd2361f6e,
and there still seem to be a couple of projects out there that require
them, as they can be more efficient than nodeRead(). Note that all
these functions are only called in readfuncs.c in terms of the core
code.

A follow-up patch plans to improve the in-core support of pg_strtok(),
making it thread-safe. This change removes the need to expose internals
specific to node reads in nodes.h, which is a header with a more general
purpose than readfuncs.h.

Based on a suggestion by me.

Author: Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>
Discussion: https://postgr.es/m/aofW6ntfWJjk_yBL@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/89d091f602fa59326d7cdf51e134e5525ef80977

Modified Files
--------------
src/include/nodes/nodes.h | 8 +-------
src/include/nodes/readfuncs.h | 6 ++++++
2 files changed, 7 insertions(+), 7 deletions(-)