pgsql: Improve castNode notation by introducing list-extraction-specifi

Improve castNode notation by introducing list-extraction-specific variants.

This extends the castNode() notation introduced by commit 5bcab1114 to
provide, in one step, extraction of a list cell's pointer and coercion to
a concrete node type. For example, "lfirst_node(Foo, lc)" is the same
as "castNode(Foo, lfirst(lc))". Almost half of the uses of castNode
that have appeared so far include a list extraction call, so this is
pretty widely useful, and it saves a few more keystrokes compared to the
old way.

As with the previous patch, back-patch the addition of these macros to
pg_list.h, so that the notation will be available when back-patching.

Patch by me, after an idea of Andrew Gierth's.

Discussion: /messages/by-id/14197.1491841216@sss.pgh.pa.us

Branch
------
REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/be182d57021033a55319ab5de737fe2294106ae0

Modified Files
--------------
src/include/nodes/pg_list.h | 7 +++++++
1 file changed, 7 insertions(+)

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