CI/CD: Only use the latest stable version of Clippy.

It's too tedious to keep the code compatible with older clippy and newer
clippy, especially if/when we enable pedantic lints.

.github/workflows/ci.yml

@@ -24,23 +24,16 @@ jobs:
 
     runs-on: ubuntu-18.04
 
-    strategy:
-      matrix:
-        rust_channel:
-          # MSRV
-          - 1.37.0
-          - stable
-
     steps:
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: ${{ matrix.rust_channel }}
+          toolchain: stable
           profile: minimal
           components: clippy
 
       - uses: actions/checkout@v2
 
-      - run: cargo +${{ matrix.rust_channel }} clippy --all-features ---all-targets -- --deny warnings
+      - run: cargo clippy --all-features ---all-targets -- --deny warnings
 
   audit:
     # Don't run duplicate `push` jobs for the repo owner's PRs.

src/arithmetic/bigint.rs

@@ -263,11 +263,7 @@ impl<M> Modulus<M> {
 
         // n_mod_r = n % r. As explained in the documentation for `n0`, this is
         // done by taking the lowest `N0_LIMBS_USED` limbs of `n`.
-        //
-        // TODO: When we stop requiring clippy 1.37.0 to accept the code, clean
-        // up these `#[allow(...)]`s.
-        #[allow(renamed_and_removed_lints, clippy::unknown_clippy_lints)]
-        #[allow(clippy::identity_conversion, clippy::useless_conversion)]
+        #[allow(clippy::useless_conversion)]
         let n0 = {
             extern "C" {
                 fn GFp_bn_neg_inv_mod_r_u64(n: u64) -> u64;