Add cargo fmt to pre-commit githook

To save devs getting frustrated by CI; add a call to `cargo +nightly
fmt` to our git pre-commit hook.

githooks/pre-commit

@@ -47,3 +47,6 @@ git diff-index --check --cached $against -- || exit 1
 
 # Check that code lints cleanly.
 cargo clippy --all-features -- -D warnings || exit 1
+
+# Check that there are no formatting issues.
+cargo +nightly fmt --check || exit 1