Cannot have comments in `@PCode`

The following code fails with the following panic.

```
@PCode {
    // hello
    Push 0
    Trace
}
```

```
thread 'main' ([...]) panicked at [...]/rascal-0.3.5/src/internal/as2_codegen/statement.rs:110:37:
error: invalid action name
  ╭▸ inline pcode
  │
```

It would be nice if:
1. the error message was more specific,
2. comments were supported in `@PCode` blocks.