Teach query_tree_walker, query_tree_mutator, and SS_finalize_plan to

Teach query_tree_walker, query_tree_mutator, and SS_finalize_plan to
process function RTE expressions, which they were previously missing.
This allows outer-Var references and subselects to work correctly in
the arguments of a function RTE.  Install check to prevent function RTEs
from cross-referencing Vars of sibling FROM-items, which doesn't make
any sense (if you want to join, write a JOIN or WHERE clause).