Fix checking for bitstring instance

For the following spec:

```
-spec foo() -> <<_:16>>.
foo() ->
    <<"ab">>.
```

`check_typespecs` was failing, due to invalid assumption in `Unit` guards, with:

```
An exception was raised: error:badarith.
Stacktrace: [{proper_typeserver,is_instance,4,
                                [{file,"src/proper_typeserver.erl"},
                                 {line,1318}]},
             {proper_typeserver,'-apply_spec_test/5-fun-0-',6,
                                [{file,"src/proper_typeserver.erl"},
                                 {line,509}]},
             {proper,apply_args,3,[{file,"src/proper.erl"},{line,1359}]},
             {proper,child,3,[{file,"src/proper.erl"},{line,1398}]},
             {proper,'-spawn_link_migrate/1-fun-1-',2,
                     [{file,"src/proper.erl"},{line,675}]}].
```