giant refactor: immutable services and impl traits

- use 'impl Future' returns for trait contracts: prevents hidden requirments
- refactor tower design around immutable services: easier to reason and use,
  more natural to use like this, makes it also more clear that user needs to handle state properly,
  prevents state keeping in service when not required and easier to integrate
  with other code bases in ecosystem (e.g. hyper)

Cargo.toml

@@ -2,7 +2,6 @@
 
 members = [
   "tower-async",
-  "tower-async-bridge",
   "tower-async-http",
   "tower-async-layer",
   "tower-async-service",

README.md

@@ -61,7 +61,7 @@ Tower async was featured as crate of the week in "This week in Rust #505": <http
 >     type Response = S::Response;
 >     type Error = S::Error;
 > 
->     async fn call(&mut self, request: Request) -> Result<Self::Response, Self::Error> {
+>     async fn call(&self, request: Request) -> Result<Self::Response, Self::Error> {
 >         tokio::time::sleep(self.delay).await;
 >         self.inner.call(request)
 >     }

justfile

@@ -0,0 +1,27 @@
+fmt:
+	cargo fmt --all
+
+sort:
+	cargo sort --workspace --grouped
+
+lint: fmt sort
+
+check:
+	cargo check --all --all-targets --all-features
+
+clippy:
+	cargo clippy --all --all-targets --all-features
+
+clippy-fix:
+	cargo clippy --fix
+
+doc:
+	RUSTDOCFLAGS="-D rustdoc::broken-intra-doc-links" cargo doc --all-features --no-deps
+
+hack:
+	cargo hack check --each-feature --no-dev-deps --workspace
+
+test:
+	cargo test --all-features
+
+qa: lint check clippy doc hack test