When analyzing a WHERE clause, we want to separate individual
factors (usually relations), and later partition them into
partition key, clustering key, and regular column relations. The
first step is separation, for which this helper is added.
Currently, it is not required since the grammar supplies the
expression in separated form, but this will not work once it is
relaxed to allow any expression in the WHERE clause.
A unit test is added.
8085b9f57a