Remove print!/println! code from mmtk-core

[qinsoon]

This pull request removes print! and println! from our code base. print!/println! are replaced with logging lines, or eprint!/eprintln! if we really need to report errors. This closes #654.

[wks]

A more general solution is to write to a &mut impl Write. It can write to a string in this way, and also a file, too, if the user desires. The Write trait still allows us to use the write! or writeln! macros for formatting, like the print! macro in the old code.

Example:

use std::fmt::{Error, Write};

fn print_something(out: &mut impl Write) -> Result<(), Error> {
    writeln!(out, "Hello world!")?;
    writeln!(out, "Number: {}", 42)?;

    Ok(())
}

fn main() {
    let mut s = String::new();
    print_something(&mut s).unwrap();
    println!("{}", s);
}

[qinsoon]

Good idea. I changed the method. However, as we cannot have methods with generic type parameters (including impl T) in a object-safe trait (Space), I moved the method outside the trait.

[wks]

LGTM