sl/symstate: remove an invalid assumption

... that was causing false alarms due to forced three-way joins
sl/symutil: improve input validation of nextObj()
sl/symutil: do not treat TS_INVALID as error

... in canPointTo{Front,Back}()
sl/adt_op_meta: ignore non-pointer fields in diffSetField()
sl/fixed_point_rewrite: display insns being removed