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

schemas/util.sql, vegbien.sql: do not declare any SQL-language functions as STRICT, because this prevents them from being inlined (as documented in the util schema comment). the STRICT qualifier has been preserved where its behavior is necessary, rather than just used for optimization.

fix: schemas/util.sql: schema comment: indicate that all SQL-language functions should never be declared STRICT, not just IMMUTABLE ones, since inlining is not limited to IMMUTABLE (or STABLE) functions (although constant folding is)