Gadt

- get_names and post_names are now invariant
- suffix in not part of the GATD anymore
```
type ('value,'name) params_type_ = GADT HERE
type ('a,+'suff,'an) params_type = ('a,'an) params_type_
``` 
There is `type 'a ocaml = string` latter, and that's the one exported. Maybe remove one of them ?