Package dependency solver

A package dependency resolver based on a SAT solver. This was originally written for the 0install package manager, but is now generic and is also used as a solver backend for opam. The SAT solver is based on MiniSat (http://minisat.se/Papers.html) and the application to package management is based on OPIUM (Optimal Package Install/Uninstall Manager). 0install-solver uses a (novel?) strategy to find the optimal solution extremely quickly (even for a SAT-based solver).