Stop exposing `less_than` and `are_equal` for automata and expressions
in dyn: rather, expose only `compare` and let the layers on top of dyn
deals with adding sugar.

The reason is simple: avoid additional precompilations for trivial
algorithms.

* vcsn/algos/compare-automaton.hh, vcsn/algos/compare-expression.hh
(less_than, are_equal): Remove the bridges.
(compare): New bridges.
* vcsn/dyn/algos.hh (compare): New.
* vcsn/dyn/algos.hxx: New, providing `less_than` and `are_equal` as thin
layers on top of `compare`.
* vcsn/ctx/instantiate.hh: Adjust.

* python/vcsn_cxx.cc: Bind only `compare`, not `less_than` and `is_equal`.
* python/vcsn/automaton.py, python/vcsn/expression.py: Use then to
define __lt__ and the like.
* tests/python/less-than.py: Check them.