Use trait objects internally to avoid generic instantiation

[smklein]

Keep the external interface to async transactions mostly unchanged, but delegate to internal functions that (as much as possible) avoid using generic functions.

This is intended to reduce code generation. For a debug build of omicron-nexus, this reduces the (debug build, Linux) binary size from 514 MiB to 497 MiB.

Source for idea: https://matklad.github.io/2021/09/04/fast-rust-builds.html#Keeping-Instantiations-In-Check

A... common case here are closures: by default, prefer &dyn Fn() over impl Fn(). Similarly to paths, an impl-based nice API might be a thin wrapper around dyn-based implementation which does the bulk of the work.

[steveklabnik]

makes sense to me. wild how much size this cuts off.

i sorta wish the compiler could do this transformation itself but ALSO compile times are long enough without adding more analysis passes, heh