Rename print!()/println!() to printf!()/printfln!()

The new names make it obvious that these generate formatted output.

Add a one-argument case that uses %? to format, just like the other
format-using macros (e.g. info!()).
auto merge of #7779 : kballard/rust/print-macro-args, r=alexcrichton

The new names make it obvious that these generate formatted output.

Add a one-argument case that uses %? to format, just like the other
format-using macros (e.g. info!()).