fix struct's guard (values of guard from super instances)
Merge branch 'master' of https://github.com/samth/pycket
add test for struct's super type guard values (skipped for now, guard_super_values in values_struct is commented, because it breaks import of contracts library)
add support of failure-result for struct's properties
add some more prims needed for struct-test.rkt
Merge branch 'master' of https://github.com/samth/pycket