Generated helpers methods for SearchForm's fields

This PR allows form's fields to be set using nice `search_form.field(value)` syntax instead of forcing to relay to the boring `search_form.set('field', value)` syntax.

These helper methods are only created when it's safe to, so if some weird field names are returned at a certain point, the code will still behave correctly (but the users will have to use the `#set` method).