Update MSRV to Rust 1.56 and use the 2021 edition

[romac]

Crate

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Summary

Because some our dependencies' MSRV is 1.56, we have to update ours to match that, which means that we can now opt-in into the 2021 edition and unlock some new features: https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html#rust-2021

Problem Definition

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