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/macros.rs

@@ -355,6 +355,6 @@ macro_rules! local_data_key(
 /// error if the value of the expression is `Err`. For more information, see
 /// `std::io`.
 #[macro_export]
-macro_rules! if_ok(
+macro_rules! try(
     ($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
 )