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)

bugfix: schemas/util.sql: join_strs_transform(): added back STRICT qualifier because it must be declared STRICT to use the special handling of STRICT aggregating functions

schemas/util.sql: %==(anyelement, anyelement): needs to be declared STABLE instead of IMMUTABLE because it depends on the search_path (as described at http://vegpath.org/links/#PostgreSQL:%20Documentation:%209.3:%20Function%20Volatility%20Categories%20**)