Curve trait for general interoperation

[mweatherley]

This is a draft PR containing a partial implementation of the curve API described in this RFC.

Objective

Create a trait, Curve<T>, providing a general functional API surface for working with curves of arbitrary origin.

Note that this is only a partial implementation which, in particular, excludes the associated #[derive] macro for Interpolable and all implementations of the Curve<T> trait for existing types.