Move axum crate into workspace subfolder

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.

.github/workflows/CI.yml

@@ -122,6 +122,16 @@ jobs:
         command: test
         args: --all-features --doc
 
+  compare-readmes:
+    needs: check
+    runs-on: ubuntu-latest
+    steps:
+    - name: README.md and axum/README.md are identical
+      uses: actions-rs/cargo@v1
+      with:
+        command: diff
+        args: README.md axum/README.md
+
   deny-check:
     name: cargo-deny check
     runs-on: ubuntu-latest