Merge: Remove niti hacks on NativeString and NativeFile

Previously, the interpreter used a FlatBuffer to implement NativeString and a File to implement NativeFile. This caused issues with applications using (mostly) `is_address_null` which could not be implemented correctly with these implementations.

This PR uses a real NativeString and NativeFile to implement the same class. For NativeFile, it is handled by the FFI so removing the old code is enough. For NativeString, many of its methods are intern so they had to be updated.

Pull-Request: #1543
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>