kv: remove Clone requirement for iterators, declutter lifetimes

[npry]

Stacked on #208

Nerd-sniped me on the iterator requiring Clone (first commit), but in the course of that, I thought that the API (mostly in operations) could be simpler if the traits used associated types more rather than type params (second commit).

The first commit does the Clone bound on the iterator by doing much more exhaustive lifetime accounting — it separately tracks the 'store, 'guard, 'tx, and 'r (ref-to-txn-or-guard) lifetimes and imposes a strict containment in that order everywhere. This is overkill — the second commit rolls it back because the API changes obviate the need — but that lets the inner iterator be held for a much more specific lifetime and avoids the need to Clone.

The second commit is an observation mainly that you can get the Storage (and index base table) from the {Table,Index}Desc rather than passing it through everywhere in parallel by adjusting how the associated types work a bit and writing a trait to wrap Storage. There's a bit of a sacrifice in verbosity in getting some ATs back out (a lot of <Desc as TableDesc>::Key), but this removes the need for the 'a lifetime nearly everywhere and makes schema::GeneratedStorage mostly implicit. Most of this is just tracking that through and being more surgical with the lifetimes