unpack::<N> should fail typecheck on rank-M (M != N) shapes

The problem is [here](https://github.com/hellas-ai/catgrad/blob/master/catgrad/src/abstract_interpreter/eval.rs#L58):

```rust
// write each result into state at op.targets ids
for ((node_id, _), result) in ssa.targets.iter().zip(results) {
on_write(*node_id, &result);
if state.insert(*node_id, result).is_some() {
return Err(InterpreterError::MultipleWrite(*node_id));
}
}
```

Namely: rust's `zip` is silently truncating one of the iterators.

**Fix**: insert a length check that there are exactly as many results as targets.