Export crates used in public APIs

This is something I've run into many times. For example, I sometimes need to add an explicit dependency on `num` or `num_traits` in order to use `num::Zero` in generic contexts.

This issue is a reminder for us to go through the public API and export all crates that appear.

The issue identifies the public API and `num`/`num_traits` as starting points. Audit exposed types and signatures for crates that users must currently depend on directly, then verify the completed API no longer requires those extra dependencies. Done means all crates appearing in the public API are exported.

Written by the indexing model from the issue text.