Update stable Rust version. (#35)

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ env:
   # version like 1.70. Note that we only specify MAJOR.MINOR and not PATCH so that bugfixes still
   # come automatically. If the version specified here is no longer the latest stable version,
   # then please feel free to submit a PR that adjusts it along with the potential clippy fixes.
-  RUST_STABLE_VER: "1.77" # In quotes because otherwise (e.g.) 1.70 would be interpreted as 1.7
+  RUST_STABLE_VER: "1.78" # In quotes because otherwise (e.g.) 1.70 would be interpreted as 1.7
   # The purpose of checking with the minimum supported Rust toolchain is to detect its staleness.
   # If the compilation fails, then the version specified here needs to be bumped up to reality.
   # Be sure to also update the rust-version property in the workspace Cargo.toml file,