Rework rules handling

Rework rules handling

Instead of having a bastard solver_addrule which wither takes an offset
into whatprovides or a direct literal, we now have two functions:
    solver_addrule2 will add a rule with one or two literals
    solver_addrulen will add a rule specified by an offset into whatprovides

This should make the code cleaner, faster, and less memory is used also.

This commit also adds a pool_ids2whatprovides function.