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.