FreePreorder should not use ObExpr and HomExpr

As Nate Osgood as shown over Zulip, making the symbolic model use a `HomExpr` comes with the assumption that the theory has `dom` and `codom` available (this gets used in the `show` method for the hom generators). This assumption is not valid for `ThPreorder` which has `lhs` and `rhs` as its accessors, so there is an error when trying to show them.

A possible solution is to just make the symbolic model use GATExprs.