[02]: Solver report

The return value of the solver part of `evanix` should contain enough information to generate a report saying (0) how many derivations in total will be built (in future versions also how many compute hours we expect to expend), (1) which "targets" will be built, (2) which "targets" will remain unsatisfied. Optional features: print how much extra budget is required to satisfy one more target. Both `--dry-run` and the normal mode should be able to produce a report like that. The return value of the solver should probably be an immutable structure, and the "solver" should be a simple enough function to be used in a unit test