Over time I imagine we're gonna have other crates in this repo that
provide utilities or integrations for axum. This prepares for that by
moving the main axum crate into its own folder.
The README situation is a bit annoying because we want `./README.md`
for viewing the repo on github but `axum/README.md` for crates.io. For
now I've just copy/pasted it and added CI step to make sure they're
identical.
This way there is now only one way to create a router:
```rust
use axum::{Router, handler::get};
let app = Router::new()
.route("/foo", get(handler))
.route("/foo", get(handler));
```
`nest` was changed in the same way:
```rust
use axum::Router;
let app = Router::new().nest("/foo", service);
```
David Pedersen