schemas/util.sql: set_comment(): accept anything with a mk_set_comment()

fix: schemas/util.sql: check_constraint_expr_disable(): need to use CASE instead of `true OR ` because OR always evaluates both sides, slowing down constraint evaluation

fix: schemas/util.sql: check_constraint view: conrelid: don't cast to regclass so can use index scan

bugfix: derived_col_constraint_initially_enabled(): use whether existing constraint is already enabled, not whether expr is unchanged (which does not affect the enabled status)

schemas/util.sql: check_constraint_canon_sql(): use util.debug_print_return_value() to assist debugging

bugfix: schemas/util.sql: check_constraint_replace(): need to use util.check_constraint_canon_sql() instead of util.canon_sql()

schemas/util.sql: added check_constraint_canon_sql(). this must be used instead of plain util.canon_sql() because unlike queries, CHECK constraints are not simplified, just standardized.

schemas/util.sql: check_constraint_replace(): support not canon-ing def.expr

bugfix: schemas/util.sql: CHECK constraint exprs: need to remove the enclosing () to be consistent everywhere

bugfix: schemas/util.sql: canon_sql(): need to include the enclosing () because the DB includes them