`Gate.to_unitary_mut(0)` overflows and panics

This code panics:

```python
from quil.instructions import Gate, Qubit
Gate("X", (), (Qubit.from_fixed(0),), ()).to_unitary_mut(0)
```

Running that with a debug build gives something like:

```
attempt to subtract with overflow
```

In a release build, I see this:

```
ndarray: inputs 0 × 0 and 1 × 1 are not compatible for matrix multiplication
```