Turn interpoli into a multi-crate repository.

With `Composition` and other such work being integrated, the simple and self-contained interpolation functions and types should probably be isolated from this more advanced feature set.

Start by reviewing the current repository structure and the code surrounding Composition and the existing interpolation functions and types. Determine a crate split that isolates the simple, self-contained interpolation functionality from the more advanced feature set. Done means the repository is organized as multiple crates with that separation preserved.

Written by the indexing model from the issue text.