Fix release metadata and repair the benchmarks

- repository pointed at the old host and the old repo name; this is the link
  crates.io and docs.rs display
- the readme example imported `differential_equations`, so it could not
  compile under the crate's published name
- `cargo bench` did not build: `Solution` borrows the ODE, so `.solve()`
  cannot be called on a temporary `Problem`. Bind it first.

readme.md

@@ -1,4 +1,4 @@
-# DifferentialEquations
+# ordinary-diffeq
 
 A library, written in Rust, for integrating ordinary differential equations. For now, this is
 relatively simple, but it does have key features that are needed for orbit propagation, ray tracing,
@@ -47,7 +47,7 @@ pendulum problem):
 
 ```rust
 use nalgebra::Vector2;
-use differential_equations::prelude::*;
+use ordinary_diffeq::prelude::*;
 use std::f64::consts::PI;
 
 // Define the system (parameters, derivative, and initial state)