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_
``` 
I think we can get rid of the option here. The only place a None is produced is in `wrap_param_type`, but the only places where the filter is actually looked at are [here](https://github.com/ocsigen/eliom/pull/112/files#diff-d38ac253f0134094227473e89d6a1342R140) and [here](https://github.com/ocsigen/eliom/pull/112/files#diff-d38ac253f0134094227473e89d6a1342R250), and they fail on None anyway.

We would just replace `TTypeFilter (t, _) -> TTypeFilter (t, None)` by `TTypeFilter (t, _) -> t` in `wrap_param_type`.