Migrate to 2018 edition and update dependencies

[fkettelhoit]

This PR migrates to the 2018 edition and updates all dependencies in a way that no duplicate dependencies are pulled in, which resolves the issues #798 and #801.

Before the PR:

$ cargo tree --duplicates | wc -l
398

After:

$ cargo tree --duplicates | wc -l
0

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Make sure these boxes are checked! 📦✅

• [x] You have the latest version of rustfmt installed

$ rustup component add rustfmt

• [x] You ran cargo fmt on the code base before submitting
• [x] You reference which issue is being closed in the PR text

✨✨ 😄 Thanks so much for contributing to wasm-pack! 😄 ✨✨

[drager]

@fkettelhoit Thanks a lot! I will go through this more closely in a while, I think this will be released in v0.11.0. Feel free to rebase master into your branch when you got time :blush:

[bytesnail]

Migrate to edition 2021 may be another choice

[saks]

Any help needed with this?