Improve MOP safety (#1182)

Specifically, `class-slot-offset` and `find-method` need to be immune to
type errors for symbolic table references (which doesn't check
anything).

Also normalizes the behavior of `class-instance?`.

Impetus:
```
Gerbil v0.18.1-72-gb280d037 on Gambit v4.9.5-124-g6d1a9a9b
> (defclass Point (x y))
> (def a (Point 1 2))

Process scheme segmentation fault (core dumped)
```

Not good UX.