Partial derivatives with `Val` data types

Expressions with values types Val<_, _> can now be converted into their partial derivatives by calls to partial et.al..

• [Breaking] Val floats, ints, and bools will be converted into one another without error. This was different before.
• [Breaking] removed find-operator-functionality from the public interface.

Full Changelog: v0.17.5...v0.18.0