mold 3.0 high-speed linker rewritten in Rust

The high-speed mold linker has reached version 3.0, the first release built on a new Rust implementation, while 2.42.1 remains the final C++ release. Command-line options, target architectures and linking performance are unchanged, but building from source now uses Cargo and requires Rust 1.95 or newer.

• mold 3.0 is the first Rust-based release; 2.42.1 ended the C++ line
• Build system moved from CMake to Cargo, requiring Rust 1.95+
• Linking performance stays on par with the previous C++ version
• Fixes cover static linking crashes and RISC-V and LoongArch issues