Prepared std::sys for removal, and made begin_unwind simpler

- `begin_unwind` is now generic over any `T: Any + Send`.
- Every value you fail with gets boxed as an `~Any`.
- Because of implementation details, `&'static str` and `~str` are still
  handled specially behind the scenes.
- Changed the big macro source string in libsyntax to a raw string
  literal, and enabled doc comments there.

src/libsyntax/ext/build.rs

@@ -604,9 +604,9 @@ impl AstBuilder for @ExtCtxt {
             span,
             ~[
                 self.ident_of("std"),
-                self.ident_of("sys"),
-                self.ident_of("FailWithCause"),
-                self.ident_of("fail_with"),
+                self.ident_of("rt"),
+                self.ident_of("task"),
+                self.ident_of("begin_unwind"),
             ],
             ~[
                 self.expr_str(span, msg),