FR: automatically derive to/from index for trivial tuple-types

Here is an example:

```
public tuple instant(real timestamp_seconds)

public function instantToIndex(instant inst) returns int
    return realToIndex(inst.timestamp_seconds)

public function instantFromIndex(int index) returns instant
    return instant(realFromIndex(index))
```

Only saves 4 lines of code to automatically derive this, but would be nice!