Assigning to a comptime variable generates incorrect Hull (and fails in yule)

Reproduction:

```
contract T {
  function main() -> word {
    let n : comptime word = 5;
    n = 7;
    return n;
  }
}
```
generates the following in Hull

```
      function main () -> word {
        n := 7
        return 7
```

which fails with 

```
Type error:
Undefined variable: n
```

We should forbid mutation of comptime variables.