Improvements.

Improvements.
* Add a function to update the substitution map that avoids the expensive checks. This is used by unconstrained variable elimination.
* use a --config flag to enable the xor solver.
* Update the dependencies with functions for unconstrained elimination.

git-svn-id: https://stp-fast-prover.svn.sourceforge.net/svnroot/stp-fast-prover/trunk/stp@1144 e59a4935-1847-0410-ae03-e826735625c1