CI: Stop using actions-rs

It seems the `actions-rs` collection of github actions are
unmaintained; using the `actions-rs/toolchain@v1` action causes
deprecation warnings in Github CI; see
<actions-rs/toolchain#221>.

So:

- Replace the `toolchain` action with `dtolnay/rust-toolchain`.
- Just run cargo directly instead of using the `cargo` action.
- Get rid of the code coverage check, which was broken anyway.

.github/workflows/CI.yml

@@ -10,16 +10,10 @@ jobs:
         uses: actions/checkout@v3
 
       - name: Install stable toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: stable
-          override: true
+        uses: dtolnay/rust-toolchain@stable
 
       - name: Run cargo check
-        uses: actions-rs/cargo@v1
-        with:
-          command: check
+        run: cargo check --all-features --all-targets
 
   test:
     name: Test Suite
@@ -29,16 +23,10 @@ jobs:
         uses: actions/checkout@v3
 
       - name: Install stable toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: stable
-          override: true
+        uses: dtolnay/rust-toolchain@stable
 
       - name: Run cargo test
-        uses: actions-rs/cargo@v1
-        with:
-          command: test
+        run: cargo test --all-features --all-targets
 
   msrv-check:
     # We require at least Rust 1.45, so we can use proc macros in
@@ -51,11 +39,8 @@ jobs:
         uses: actions/checkout@v3
 
       - name: Install 1.45 toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: 1.45.0
-          override: true
+        uses: dtolnay/rust-toolchain@1.45.0
+
       # We use the git CLI here, since otherwise cargo fails with
       # "error reading from the zlib stream; class=Zlib (5)", see
       # <https://github.com/rust-lang/cargo/issues/10303>.
@@ -71,56 +56,10 @@ jobs:
         uses: actions/checkout@v3
 
       - name: Install stable toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: stable
-          override: true
-          components: rustfmt, clippy
+        uses: dtolnay/rust-toolchain@stable
 
       - name: Run cargo fmt
-        uses: actions-rs/cargo@v1
-        with:
-          command: fmt
-          args: --all -- --check
+        run: cargo fmt --all -- --check
 
       - name: Run cargo clippy
-        uses: actions-rs/cargo@v1
-        with:
-          command: clippy
-          args: -- -D warnings
-
-
-  codecov:
-    name: Code coverage
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout sources
-        uses: actions/checkout@v3
-
-      - name: Install nightly toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: nightly
-          override: true
-
-      - name: Build with instrumentation
-        uses: actions-rs/cargo@v1
-        with:
-          command: test
-          args: --package lexpr --all-features --no-fail-fast
-        env:
-           CARGO_INCREMENTAL: '0'
-           RUSTFLAGS: '-Zprofile -Ccodegen-units=1 -Cinline-threshold=0 -Clink-dead-code -Coverflow-checks=off -Cpanic=abort -Zpanic_abort_tests'
-           RUSTDOCFLAGS: '-Cpanic=abort'
-
-      # TODO: collect statistics with `gcovr`, which doesn't play with
-      # current "gcov" binary format.
-      - name: Collect coverage information
-        uses: actions-rs/grcov@v0.1
-
-      # TODO: upload coverage, see
-      # <https://github.com/marketplace/actions/codecov>
-      - name: Upload to codecov
-        uses: codecov/codecov-action@v1
+        run: cargo clippy -- -D warnings