Now we monomorphize SolverInstallPlan so that its data structures
are non-parametric, and delete functions that are not needed.
Specifically:

    - GenericPlanPackage -> SolverPlanPackage, lose the
      type parameters, lose the Processing/Installed/Failed
      constructors (this makes some partial functions total! Yay!)

    - GenericInstallPlan -> SolverInstallPlan

    - PlanProblem -> SolverPlanProblem

    - Deleted ready, processing, completed, failed,
      preexisting

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>