Broken `is` tests on CCL.

The `is` tests for class and condition need to be changed to

```
(and (symbolp symbol) (find-class symbol nil) (not (is symbol :condition)))
```

and

```
(and (symbolp symbol) (find-class symbol nil) (subtypep (find-class symbol nil) 'condition))
```

respectively.

CCL will error if the first argument to `find-class` is not a symbol.
