pgsql: Refactor pl_funcs.c to provide a usage-independent tree walker.

Refactor pl_funcs.c to provide a usage-independent tree walker.

We haven't done this up to now because there was only one use-case,
namely plpgsql_free_function_memory's search for expressions to clean
up. However an upcoming patch has another need for walking plpgsql
functions' statement trees, so let's create sharable tree-walker
infrastructure in the same style as expression_tree_walker().

This patch actually makes the code shorter, although that's
mainly down to having used a more compact coding style. (I didn't
write a separate subroutine for each statement type, and I made
use of some newer notations like foreach_ptr.)

Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Reviewed-by: Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com>
Discussion: https://postgr.es/m/CACxu=vJaKFNsYxooSnW1wEgsAO5u_v1XYBacfVJ14wgJV_PYeg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6a7283dd2f1c573f2cf3daaf14e8a246577da57d

Modified Files
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src/pl/plpgsql/src/pl_funcs.c | 582 ++++++++++++++++++------------------------
1 file changed, 244 insertions(+), 338 deletions(-)