fix: schemas/util.sql: diff(regclass, regclass): usage: col_type_null must actually be a shared base type of the tables

fix: schemas/util.sql: diff(regclass, regclass): usage: text arguments must be cast to regclass to bind the right variant of diff()

fix: schemas/util.sql: diff(regclass, regclass): renamed params to *_table because these actually can't be views, since views don't support inheritance

schemas/util.sql: added copy_struct()

schemas/vegbien.sql: added remake_diff_table()

schemas/util.sql: added inherit()

bugfix: schemas/util.sql: diff(): need to explicitly cast each side to the return type because this does not happen automatically even when an implicit cast is available

schemas/util.sql: renamed diff_views(), diff_any() to diff(...) because they are overloads of the same operation

schemas/util.sql: diff_any(): documented how to run EXPLAIN on the FULL JOIN query

schemas/util.sql: removed non-useful diff_queries() (it is not possible to match distinct `record` types to a common return type, so this would not have worked). use diff_views() instead.