Modernize with C++ 20 Concepts

We need concepts for lookup callbacks, comparable types for heterogeneous lookups, and even the supported functionality of entire collections to feature-gate locked and partitioned wrappers.

Start by locating the lookup callback and heterogeneous lookup implementations, then inspect the APIs for locked and partitioned wrappers. Determine how C++20 concepts should express those constraints and feature gates; done means the supported functionality is constrained consistently across these areas and the relevant tests pass.

Written by the indexing model from the issue text.