Demonstrate multiple impl blocks to the same struct are possible

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Should be mentioned in the "Method Syntax" chapter: https://doc.rust-lang.org/stable/book/method-syntax.html

Example: https://play.rust-lang.org/?gist=d3c7bd9cb01767d517ed&version=stable

Rationale: Makes the following traits section more sense: https://doc.rust-lang.org/stable/book/traits.html#inheritance