make PolylineError clonable

[michaelkirk]

• I agree to follow the project's code of conduct.
• I added an entry to the project's change log file if knowledge of this change could be valuable to users.
  • Usually called CHANGES.md or CHANGELOG.md
  • Prefix changelog entries for breaking changes with "BREAKING: "

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Mostly because "why not", but specifically so I can use it with thiserror #[from], e.g.

#[derive(Debug, Error, Clone, PartialEq)]
enum MyError {
    #[error("Polyline error: {0}")]
    Polyline(#[from] PolylineError)
    #[error("Foo error: {0}")]
    Foo(#[from] FooError)
}