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.

bugfix: schemas/util.sql: diff_queries(), diff_views(): embed views verbatim as FROM items, so that the type of their corresponding columns is the view's rowtype. this is necessary in order to match the types of the columns to the return type (set by col_type_null).

schemas/util.sql: diff_cols(): renamed to diff_queries() because multiple columns are supported

fix: schemas/util.sql: diff_cols(): don't assume that each side will have one column, as many of our validations queries have multiple columns

bugfix: schemas/util.sql: diff_views(): need to use $2, not $1, for the 2nd query

fix: schemas/util.sql: materialize_query(): remove any extra blank line before the query, to improve readability when newlines are used as a log message separator and are therefore confusing in the middle of a log message