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()

fix: schemas/util.sql: eval2*(): added search_path function option in order to limit the effects of any `SET LOCAL search_path` in the invoked query to the current function. however, plain eval() is not changed because it is often used to execute a `SET LOCAL search_path` in the calling function.

schemas/util.sql: diff(text, text): removed unnecessary casts to the base type in the join condition and the WHERE filter. these had been presumed necessary due to errors, but the errors turned out to be caused by the operator not being in the search_path. note that the casts in the columns list are still needed, as described in the associated comment.

schemas/util.sql: materialize_query(): add a comment on the table with the query it was generated from