Release 0.8.0

Scalar now inherits num-dual's DualNum<Primitive = f64> and
DualStruct<Real = f64> and declares only lift. re comes from DualStruct.
A blanket impl replaces impl Scalar for f64 and the impl_scalar! macro
over the dual types, so Scalar covers f64 and every num-dual dual type
and a sibling crate built on num-dual can share one bound.

num-dual is a plain dependency rather than optional.

Breaks: the autodiff feature is removed, so a dependent naming it in its
feature list no longer resolves. Scalar's blanket impl makes any manual
impl of it a conflicting implementation, and Scalar no longer declares
re, so a qualified Scalar::re(&x) call must become x.re().

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ordinary-diffeq"
-version = "0.7.0"
+version = "0.8.0"
 edition = "2021"
 authors = ["Connor Johnstone"]
 description = "A library for solving differential equations based on the DifferentialEquations.jl julia library."