Merge: Implementation of some mechanisms with direct access in nitvm

The method dispatch in nitvm is now implemented with direct access, i.e. without perfect hashing.

This implementation is faster than perfect hashing, but just work in a few cases.
Subtyping test is also implemented with direct access, but it is not used yet in runtime mechanisms because the guard would be to complex and will decrease performances.

Pull-Request: #976
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>