* vcsn/algos/derived-term.hh: Implement lazy support.

* lib/vcsn/dyn/context-parser.cc, lib/vcsn/dyn/context-printer.cc:
Add support for derived_term_automaton.

* tests/bin/test.py (CHECK_NE): New.
* tests/python/derivation.py: Check lazy construction.

* libexec/vcsn-score: Bench lazy construction.