Add support for SQL::QueryMaker

As of now, the only way to define SQL expressions in SQL::QueryMaker is to use certain forms of unblessed references.  For example, developers need to write like `foo: [ '<' => $v ]` to use the `<` operator, which IMO is difficult to use / understand.  On the contrary, SQL::QueryMaker is a module that export functions to define such expressions more expressively.

This PR adds support for SQL::QueryMaker to SQL::Maker; i.e. the expression objects generated by SQL::QueryMaker (or any other object that exports `as_sql` and `bind` methods) are recognized by SQL::Maker.

Once the PR gets merged, the users of SQL::QueryMaker can choose either way to define SQL expressions.