We do have dyn::are_equivalent for expression, but not on the
templated level.

* vcsn/algos/are-equivalent.hh: Do it.
Use derived-term, instead of standard, as what the case before.
* vcsn/dyn/algos.hh, lib/vcsn/algos/are-equivalent.cc: Adjust.
* tests/python/is-equivalent.py: Check a weighted case.