Completely remove the capsule and cross

[mohanson]

• Completely remove the capsule and cross. Now all test/example contracts are written directly in examples, build it with RUSTFLAGS="-C target-feature=-a" cargo build --target riscv64imac-unknown-none-elf --features="build-with-clang,dummy-atomic" --example exec_caller_by_code_hash
• Upgraded clang to clang-18. Since capsule and its docker environment is removed, you must install clang-18 or riscv64-unknown-elf-gcc locally.