Made fail! and assert! accept both &'static str and ~str, as well as a fmt! like format list.

Unwinding through macros now happens as a call to the trait function `FailWithCause::fail_with()`, which consumes self, allowing to use a more generic failure object in the future.

src/test/run-fail/assert-eq-macro-fail.rs

@@ -1,4 +1,4 @@
-// error-pattern:expected: 15, given: 14
+// error-pattern:left: 14 != right: 15
 
 #[deriving(Eq)]
 struct Point { x : int }