rustfmt.toml: init

When using rustic-mode + Emacs, saving in a file that has the async keyword pops up an error box because the edition isn't specified for rustfmt itself. Adding a rustfmt.toml file with edition = "2018" lets rustfmt know that the codebase is indeed 2018-edition Rust.

Created by  cole-h  on November 16, 2020
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