Replace unidiomatic 'extern crate' to 'use x as _'

[hyperslv]

Replacing unidiomatic extern crate in case of importing of panic_ crates.
The appropriate PR to https://github.com/rust-embedded/book/ is here.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @therealprof (or someone else) soon.

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[adamgreig]

Thanks!