std: Use concat! and stringify! to simplify the most common assert! case.

With no custom message, we should just use concat! + stringify! for
`assert!(expr)`.

Inspired by issue #16625

src/libstd/macros.rs

@@ -96,7 +96,7 @@ fn _run_fmt(fmt: &::std::fmt::Arguments) -> ! {
 macro_rules! assert(
     ($cond:expr) => (
         if !$cond {
-            fail!("assertion failed: {:s}", stringify!($cond))
+            fail!(concat!("assertion failed: ", stringify!($cond)))
         }
     );
     ($cond:expr, $($arg:expr),+) => (