Allow any type of argument for one-argument print! invocations

[Manishearth]

Currently, if one wants to print a number (or any other non-string value), one has to use println!("{}", 1234), which is a tad unwieldy. println!(1234) throws an error (one-argument invocations only work with strings). It's also annoying to write during debugging.

Why not overload the macros so that println!(expr) is an alias for println!("{}", expr)? This won't break any current behavior, but it will turn an error into something useful.

I have a possible fix on this branch, though eddyb suggested I try to make the changes in format_args! itself. (I'm not so familiar with that code, so I'd prefer to get feedback here before working on that.)

[lifthrasiir]

Previously:

• Rename print!()/println!() to printf!()/printlnf!() #7779 Rename print!()/println!() to printf!()/printlnf!()
• printf! behaving inconsistently with a single argument #8094 printf! behaving inconsistently with a single argument
• Add a shorter macro for println!("{:?}", foo) #12015 Add a shorter macro for println!("{:?}", foo)
• format_args: allow omission of format string #12384 format_args: allow omission of format string

It seems to be a popular feature request with some active criticism indeed.