This branch updates the READMEs for all Tower crates.
I've added the lib.rs docs to the `tower` crate's README, and added
crates.io, docs.rs, and updated CI badges to all the crates READMEs.
Since we no longer use Azure Pipelines for CI or Gitter for chat, I've
removed those badges and replaced them with GitHub Actions and Discord
badges, respectively.
I also fixed a typo in the `tower` lib.rs docs that was breaking some of the
RustDoc links, since I noticed it after copying those docs into the README.
Signed-off-by: Eliza Weisman <eliza@buoyant.io>
This change introduces the new `tower-layer` crate and the foundational `Layer` trait to go along with it. This trait allows one to easily compose a set of `Service`s that take an inner service. These services only modify the request/response. This also provides the `Layer` implementation for many of the tower crates.
