Merge rust-bitcoin/rust-bitcoin#1419: Run clippy in ci for `hashes`

b78ba730f2 hashes: Run clippy in ci (Tobin C. Harding)
5e67f7a7cb Remove the unnecessary explicit reference (Tobin C. Harding)

Pull request description:

  Currently we only run the linter in `bitcoin/contrib/test.sh`, we should do the same in the `hashes` ci script.

  - Patch 1: Fix current clippy issues in `hashes` crate
  - Patch 2: Run clippy in CI for `hashes` crate

ACKs for top commit:
  apoelstra:
    ACK b78ba730f2
  Kixunil:
    ACK b78ba730f2

Tree-SHA512: f9fedcd8c1a06c715396cf6c7b752e2c9e32dbfde48c8b4bcb9ac5e701abe109ddeadc2e7466f6926f7c3ab74fa26e68b70473b4a5b5009cb4644d634707d4ea

hashes/contrib/test.sh

@@ -22,6 +22,11 @@ export CARGO_TERM_VERBOSE=true
 cargo build --all
 cargo test --all
 
+if [ "$DO_LINT" = true ]
+then
+    cargo clippy --all-features --all-targets -- -D warnings
+fi
+
 if [ "$DO_FEATURE_MATRIX" = true ]; then
     cargo build --all --no-default-features
     cargo test --all --no-default-features