jextract: Hide constructors (pointer, arena) because they are too easy to mistakenly call

This looks right:

```
new Data(bytes, arena)
```

but is super wrong, because this just wraps the pointer to the types, but instead we want to

```
Data.init(bytes, arena),
```

which runs the Data initializer.

We should hide the constructors and offer them via a well named "scary" method name