lic2soc: try a few optimisation wrt Soc.gao scheduling and test opening.

Indeed, in order to sort Soc.gao, I currently process in 3 stages ;
1. compute a total ordering of Actions.t according to the deps
2. transform Actions.t into Soc.gao (List.map)
3. factorize Soc.gao by looking at consecutive gao that have the same guard

With this change, between steps 1 and 2, I reorder the list of actions
so that group of independent actions are sorted according to their
clock. The idea is of course to provide to step 3 more opportunities
of factorization.

The result is not very convincing yet.

I've also tried to sort the actions wrt their clock before performing
the toposort. Seems not too bad.

I've kept the old behavior by default though.