Organize generic arguments as layouts

[frankmcsherry]

A brainstorming PR to see what it looks like to organize various walls of generic parameters behind types that provide coherent associated type guidance. Many type signatures and where clause lengths decrease. The main downside is that I struggled to maintain the constraint that O must have its TryInto::Err variants implement Debug. I didn't try too hard, but all that is lost is naming a specific number that cannot be cast should it error (it shouldn't!).

There is still all of OrdKey* to do, but I thought I'd put this up to look at it.

cc: @antiguru, @petrosagg