Fix a bug when expanding nodes (-en). A Node that calls a node that

itself calls another node on a non-trivial clock (i.e., using a when)
was not producing correct code.

I've fixed this by performing the fix-point on nodes rather than on
equations. Indeed its more natural and efficient, and it avoid the
problem above. However, I did not really fix the problem, but just
turn around it. All tests seems to work fine though.

nb : #FAILS=81->80 (and -2 unresolved, but because I fixed the prog)