Maybe try 'hint' instead of 'phase' to select decision phase (at most) once

[arminbiere]

After discussing with @aniemetz about what exactly CaDiCaL::phase (int lit) is doing, also related is #56, and negative experience on using phase with CaDiBack, the proposal is to have a CaDiCaL::hint (int lit) function which takes priority when selecting a phase in decide_phase but is reset as soon it is used once (or overwritten?).