fix: schemas/util.sql: diff(text, text): FULL JOIN SELECT statement: don't put a comment at the very beginning, because this prevents it from being autoexplained (this is the query we particularly want EXPLAIN output for)

fix: schemas/util.sql: debug_print_sql(): only surround EXPLAIN output with newlines if there is any

fix: schemas/util.sql: debug_print_sql(): need newline before and after EXPLAIN output to visually separate it from other debug info

schemas/util.sql: util.debug_print_sql(): include the EXPLAIN output of applicable queries, for easier query plan debugging (just like lib/sql.py's db.autoexplain)

schemas/util.sql: added explain2notice_msg_if_can()

schemas/util.sql: added is_explainable(sql)

schemas/util.sql: added first_word()

schemas/util.sql: explain2notice(): use util.raise_notice()

schemas/util.sql: explain2notice(): added helper function explain2notice_msg() which can also be used individually

fix: schemas/util.sql: schema comment: noted that avoiding use of STRICT also makes functions much easier to troubleshoot, because they won't mysteriously do nothing if called with only NULL arguments, even when you have added debug-print statements