Cleanup CI (#141)

* Feature-gate test that depends on non-default features

Makes `cargo check` work without extra flags.

* Don't set doc(html_root_url)

It is no longer recommended:
https://github.com/rust-lang/api-guidelines/pull/230

* Remove documentation URL from Cargo.toml

crates.io will link to the right version on docs.rs automatically.

* Ensure toolchains installed by actions-rs/toolchain are actually used

* Fix missing rustup component in check job

* Raise MSRV to 1.51

Older versions weren't actually working before.

* Only run clippy & rustfmt on stable toolchain

MSRV is checked in test-versions.

* Allow cargo doc to succeed without headers and multipart features

CI will still ensure that intra-doc links that rely on these are not broken.

src/tests.rs

@@ -1,6 +1,5 @@
 use crate::{
-    extract::RequestParts, handler::on, prelude::*, response::IntoResponse, routing::nest,
-    routing::MethodFilter, service,
+    extract::RequestParts, handler::on, prelude::*, routing::nest, routing::MethodFilter, service,
 };
 use bytes::Bytes;
 use futures_util::future::Ready;
@@ -514,8 +513,10 @@ async fn layer_on_whole_router() {
 }
 
 #[tokio::test]
+#[cfg(feature = "header")]
 async fn typed_header() {
-    use extract::TypedHeader;
+    use crate::{extract::TypedHeader, response::IntoResponse};
+
     async fn handle(TypedHeader(user_agent): TypedHeader<headers::UserAgent>) -> impl IntoResponse {
         user_agent.to_string()
     }