Crate that only makes the S3 headers.

In Rust, everyone's S3 tool is for some specific network library or async engine, so we ended up with five pages of variations. So I added to the mess by making a library that only gives the headers -- literally just an array of key/value strings. https://crates.io/crates/common-s3-headers

I'll add more use-cases as I run into them with the product I'm creating, but it'll always just do the math to construct the headers only.

Basically, I'm saying that we should really start dividing up our libraries along different boundaries.