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.

Cargo.toml

@@ -1,11 +1,11 @@
 [package]
 name = "ordinary-diffeq"
-version = "0.3.0"
+version = "0.3.1"
 edition = "2021"
 authors = ["Connor Johnstone"]
 description = "A library for solving differential equations based on the DifferentialEquations.jl julia library."
 readme = "readme.md"
-repository = "https://gitlab.rcjohnstone.com/connor/differential-equations"
+repository = "https://git.rcjohnstone.com/connor/ordinary-diffeq"
 license = "MIT"
 exclude = ["roadmap/"]