Improvements of newton_generic

This implements a few general improvements in ```newton_generic.h``` (avoiding of some unnecessary copies and recreating some data structures).
Additionally it also implements ```CommutativeConcreteLinSolver```, which computes the Kleene star at every iteration using Floyd-Warshall (i.e., no symbolic encoding, no ```make_free```, etc., but the Jacobian matrix *is* computed just once).  Combined with the above improvements the time to solve ```float_perf_test4.g``` goes down from around 90s to 1.8s (we're 50 times faster!). :-)

As always, all comments are welcome.