Add check for to-one path in joins

[henneberger]

If the table is a to-one then we cannot use it in a join, it must use the column path syntax.

For example, this is invalid if details is to-one from events:

MyTable := SELECT *
                     FROM Events c JOIN c.details d;

The workaround is this:

MyTable := SELECT c.*, c.details.*
                     FROM Events c;

We should give a good error message for this case.