Several suggestions to improve the performance of rewriters

[mlaveaux]

Currently performance of rewriting is still bottle necked due to allocations and term protections which could be avoided as followed:

• Make the semi compressed tree application use implicit protections.
• Introduce a term reassignment operator for example to optimise the subsitution recursion
• Deal better with rewriting conditions, avoiding allocating multiple rewrite stacks for instance.
• Do not construct a term for matching purposes, but introduce some kind of generic match term.