How to enable printf/log in contracts written in C

[XuJiandong]

1. set -fno-builtin-printf while compiling C code
2. Add macro CKB_C_STDLIB_PRINTF before including stdio.h.
3. With Rust test cases, should specify how to receive the log

fn debug_printer(script: &Byte32, msg: &str) {
    let slice = script.as_slice();
    let str = format!(
        "Script({:x}{:x}{:x}{:x}{:x})",
        slice[0], slice[1], slice[2], slice[3], slice[4]
    );
    println!("{:?}: {}", str, msg);
}
verifier.set_debug_printer(debug_printer);

[xxuejie]

Note that in CKB contract development, we consider it a best practice to always add -fno-builtin-printf when building C smart contracts.