Mass rename if_ok! to try!

This "bubble up an error" macro was originally named if_ok! in order to get it
landed, but after the fact it was discovered that this name is not exactly
desirable.

The name `if_ok!` isn't immediately clear that is has much to do with error
handling, and it doesn't look fantastic in all contexts (if if_ok!(...) {}). In
general, the agreed opinion about `if_ok!` is that is came in as subpar.

The name `try!` is more invocative of error handling, it's shorter by 2 letters,
and it looks fitting in almost all circumstances. One concern about the word
`try!` is that it's too invocative of exceptions, but the belief is that this
will be overcome with documentation and examples.

Close #12037

src/libstd/vec.rs

@@ -2645,15 +2645,15 @@ impl<A: DeepClone> DeepClone for ~[A] {
 
 impl<'a, T: fmt::Show> fmt::Show for &'a [T] {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        if_ok!(write!(f.buf, "["));
+        try!(write!(f.buf, "["));
         let mut is_first = true;
         for x in self.iter() {
             if is_first {
                 is_first = false;
             } else {
-                if_ok!(write!(f.buf, ", "));
+                try!(write!(f.buf, ", "));
             }
-            if_ok!(write!(f.buf, "{}", *x))
+            try!(write!(f.buf, "{}", *x))
         }
         write!(f.buf, "]")
     }