Significant update:
Binder and Data actor made independent from Processor#Elt.
The dependency was mostly fake: it allowed passing an elt as another
parameter to converters, at a cost of tying implementations.
It made the code slower (classTag to pass on, one more parameter to pass
on), for a feature that's not even sure to be ever used.

If necessary to come back on this change:
* either see if one can pass on a Tuple (keep one parameter!) ; hence
(U,E) would fit again
* or reinstall the old code, but with any parameter [E] not tied to the
core framework.