fail!() requires formatting in libcore

[bluss]

libcore's failure hooks require its users to use formatting. libcore users like all the instances of fail!() or assert!() with a simple message would generate less code if there was a format-less code path.

This already exists in the libstd fail!().

[bluss]

This is fixed, because panic!("string literal") does not insert much more than a function call to a #[cold] function now.