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/tuple.rs

@@ -161,9 +161,9 @@ macro_rules! write_tuple {
         write!($buf, "({},)", *$x)
     );
     ($buf:expr, $hd:expr, $($tl:expr),+) => ({
-        if_ok!(write!($buf, "("));
-        if_ok!(write!($buf, "{}", *$hd));
-        $(if_ok!(write!($buf, ", {}", *$tl));)+
+        try!(write!($buf, "("));
+        try!(write!($buf, "{}", *$hd));
+        $(try!(write!($buf, ", {}", *$tl));)+
         write!($buf, ")")
     });
 }