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.

.github/workflows/CI.yml

@@ -10,15 +10,14 @@ jobs:
   check:
     # Run `cargo check` first to ensure that the pushed code at least compiles.
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        rust: [stable, 1.40.0]
     steps:
     - uses: actions/checkout@master
     - uses: actions-rs/toolchain@v1
       with:
-        toolchain: ${{ matrix.rust }}
+        toolchain: stable
+        override: true
         profile: minimal
+        components: clippy
     - name: Check
       uses: actions-rs/cargo@v1
       with:
@@ -37,9 +36,12 @@ jobs:
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: stable
+        override: true
         profile: minimal
     - name: cargo doc
       working-directory: ${{ matrix.subcrate }}
+      env:
+          RUSTDOCFLAGS: "-D broken-intra-doc-links"
       run: cargo doc --all-features --no-deps
 
   cargo-hack:
@@ -49,6 +51,7 @@ jobs:
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: stable
+        override: true
         profile: minimal
     - name: Install cargo-hack
       run: |
@@ -63,12 +66,13 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: [stable, beta, nightly, 1.40.0]
+        rust: [stable, beta, nightly, 1.51]
     steps:
     - uses: actions/checkout@master
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: ${{ matrix.rust }}
+        override: true
         profile: minimal
     - name: Run tests
       uses: actions-rs/cargo@v1