schemas/util.sql: added set_search_path()

schemas/util.sql: runnable_sql(): don't output search_path before a SET statement

schemas/util.sql: added is_set_stmt()

schemas/util.sql: diff(text, text): use mk_diff_query()'s new cols param to avoid a nested SELECT

schemas/util.sql: mk_diff_query(): parameterized the customizable parts of the query, to allow them to be replaced with their EXPLAIN expansion (which is what we ultimately want, so that the query does not refer to any internal tables or views)

schemas/util.sql: diff(text, text): use util.mk_diff_query(). this splits the complex query-generation code apart from the query-execution code.

bugfix: schemas/util.sql: undid r12472 because the command needed to limit the effects of any `SET LOCAL search_path` to the current function will unfortunately also clear any existing search_path, which may be needed for the eval query to execute

schemas/util.sql: added mk_diff_query()

schemas/util.sql: debug_print_sql(): use runnable_sql()

schemas/util.sql: added runnable_sql()