Make DistroVersion more static

[mikrostew]

One issue that came out of #234 is that it should be possible to make DistroVersion more static (instead of a dynamic Enum), and verify the code correctness using the type system. This may be possible using some combination of Traits and associated types, but it was non-obvious how exactly to do that without blocking the PR for some time.

[charlespierce]

I thought about this a little after seeing Dave's comment about associated types, and the only thing that immediately came to me would be inverting the control flow a little. Currently we are set up to do Inventory::fetch(tool), passing the tool that we want to fetch into a single inventory method.

If we instead made Tool into a Trait, with an associated Distro type (which itself has an associated Version type), then we could have Tool::fetch(collection), where we pass the Inventory Collection into the tool to do the fetching. We could use the associated Distro type to ensure that we are passing a Collection of the correct kind in, and getting the correct type out.

I haven't worked through all the intricacies of that approach though, perhaps I'll take a quick stab at it once #234 is merged to see if it's feasible.