Issue #36.

* vcsn/algos/multiply.hh: When determinizing, use the result...
* vcsn/algos/sum.hh: Provide a functional version.
* tests/python/multiply.py (check_mult_deter): Remove.
Instead of duplicating code, make it more generic.
Check that the result is deterministic when it should be.
While at it, check that the result is standard when the input
is.