split: fix the handling of extended expressions
Our handling of `&`, modeled after `.`, was dead wrong. The point is to capture all the sums that appear at the top-level, so `(a?)[bc]` is split in `b⊕c⊕a(b+c)`. This did require to splut `[bc]` because `a?` is nullable. However, doing the same with `&` splits `(a?)&[bc]` into `b⊕c⊕a&(b+c)`, which is obviously wrong (e.g., the latter accepts `b` but not the former). Don't try to split extended operators. Eventually, we might decide to distribute from the left to the right, in which case `(a?)&[bc]` should be split in `\e&[bc] ⊕ a&[bc]`, but not what we had currently. See issue #149 about that. * vcsn/algos/split.hh: Avoid recreating equivalent expressions: just reuse the expression itself when there is nothing to do. * tests/bin/test.py (shortest): New. * tests/python/split.py (check): Check that the split expression is equivalent to the expression itself. Check extended operators.