Update dependencies and refactor random number generation to use `rng…

[johanneshahn]

…()instead ofthread_rng()This pull request updates the codebase to use Rust 2021 edition, modernizes dependencies, and replaces the deprecatedthread_rngfunction with therngfunction from therand` crate. Additionally, it removes unsafe code in key deserialization and includes minor refactoring for clarity. Below are the most important changes grouped by theme:

Dependency Updates

• Updated the Rust edition from 2018 to 2021 in Cargo.toml and fuzz/Cargo.toml for modern language features. [1] [2]
• Upgraded rand from version 0.5 to 0.9.0 and rand_core from version 0.2 to 0.9.3 in Cargo.toml.
• Updated libfuzzer-sys dependency from 0.3 to 0.4.9 in fuzz/Cargo.toml.

Transition to rng from thread_rng

• Replaced all instances of thread_rng with rng across the codebase for compatibility with the updated rand crate. This includes changes in src/aggsig.rs, src/ecdh.rs, src/key.rs, and fuzz/fuzz_targets/fuzz_aggsig.rs. [1] [2] [3] [4]

Removal of Unsafe Code

• Eliminated unsafe initialization of arrays in the Deserialize implementation for PublicKey in src/key.rs, replacing it with a safe zeroed array initialization. [1] [2]

Minor Refactoring

• Commented out unused try_fill_bytes methods in the test module of src/key.rs for improved code clarity. [1] [2] [3]

Documentation and Constants

• Added and clarified comments for constants related to Bulletproofs and range proofs in src/constants.rs.