Remove dependency of trie.ml and leapfrog.ml on Named_ref.

[Ekdohibs]

This cleans up the code and should make it possible to cleanly move these files in algorithms/.

[bclement-ocp]

Looks good overall, thanks! Just two minor comments.

I am also wondering if we would want to make the cell field of Parameter.t a 'a option ref with_name directly (and remove the name field entirely), but that's probably too nitpicky.