Grouping leaf and interior nodes inside arrays

[mlemerre]

This would allow fewer indirections. Furthermore while a node is not shared, the array can me modified in place.

We probably still want the unicity of representation as it is important for our algorithms.

When extracting these compound nodes in the view, we must not create big arrays, but we can use pointers to the existing big array to indicate what is the current status.

Our code is quite modular and would probably work quite well with such a representation.