Faster opt

NEWS.txt
- Faster optimization phase during compilation (Frederic B.)
   - Particulary for big graph.
- This was done in a few way including a constant cache for tensor scalar (Frederic B.)
   - This needed a change to the interface of FunctionGraph(). Now by default it clone its inputs as it can't share variable with other other FunctionGraph, even for constant. A parameter allow to don't clone the input.