ISLE: support primitive type definitions without naming the primitive type twice

Right now:

```lisp
(type (primitive ))
```

If `MyIsleType == MyRustType` then we should support just doing:

```lisp
(type MyType (primitive))
```

I just spent too much time debugging ISLE generating the "wrong" type when I copy-pasted a primitive type definition and changed the ISLE name without also changing the name inside `(primitive ...)`. This short-hand syntax, which is what I want 100% of the time thus far, would have prevented this bug from occurring.