Explicitly name selector in generated code

Currently, we generate the call data like so:

```rust
let mut data = vec![155u8, 174u8, 157u8, 94u8];
arg1.encode_to(&mut data);
...
```

To better showcase what's happening to anyone reading the generated code, we should instead do something like:

```rust
let selector = vec![155u8, 174u8, 157u8, 94u8];
let mut data = selector;
arg1.encode_to(&mut data);
...
```