Update to Rust Edition 2021

[cryptoquick]

Followed the instructions here:
https://doc.rust-lang.org/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html

Resolves this warning when trying to run the example:

warning: virtual workspace defaulting to `resolver = "1"` despite one or more workspace members being on edition 2021 which implies `resolver = "2"`
note: to keep the current resolver, specify `workspace.resolver = "1"` in the workspace root's manifest
note: to use the edition 2021 resolver, specify `workspace.resolver = "2"` in the workspace root's manifest
note: for more details see https://doc.rust-lang.org/cargo/reference/resolver.html#resolver-versions

Also, forgive the TOML autoformat noise, I didn't realize it would run when doing a global find-and-replace, but it did. At least it looks neater?

[JeremyRubin]

not sure that Zig is required, but doesn't seem like the worst idea to use it as CC. Can you say more on why?

[cryptoquick]

It was something I was told... I can try validating that assumption