chore: Maintenance and update toolchain version

[huitseeker]

• Optimized memory usage by employing efficient cloning through the clone_from method in compute_coeffs and build_frames functions.
• Included the #[allow(dead_code)] attribute to the CaseClause struct and Op trait, to suppress possible warnings about unused codes.
• Enhanced constancy in the lurk-metrics/src/lib.rs by making the LOCAL_SINK instantiation constant.
• Upgraded the Rust toolchain channel from 1.76 to 1.78 in rust-toolchain.toml.
• Improved clarity and standard compliance by renaming the .cargo/config file to .cargo/config.toml.

FIxes #1232