refactor: convert Results to immutable named tuples

[ferringb]

Slotting is both faster in access (drastically), faster in pickling, and simplifies freezing which would prevent code mistakes mutating the results during consumption.

The point of this is more to force defensive measures into the implementation; the perf gain is ancillary but welcome, even if I expect it to be lost in the background of how much work pkgcheck does.