Rename fail! to panic!

https://github.com/rust-lang/rfcs/pull/221

The current terminology of "task failure" often causes problems when
writing or speaking about code. You often want to talk about the
possibility of an operation that returns a Result "failing", but cannot
because of the ambiguity with task failure. Instead, you have to speak
of "the failing case" or "when the operation does not succeed" or other
circumlocutions.

Likewise, we use a "Failure" header in rustdoc to describe when
operations may fail the task, but it would often be helpful to separate
out a section describing the "Err-producing" case.

We have been steadily moving away from task failure and toward Result as
an error-handling mechanism, so we should optimize our terminology
accordingly: Result-producing functions should be easy to describe.

To update your code, rename any call to `fail!` to `panic!` instead.
Assuming you have not created your own macro named `panic!`, this
will work on UNIX based systems:

    grep -lZR 'fail!' . | xargs -0 -l sed -i -e 's/fail!/panic!/g'

You can of course also do this by hand.

[breaking-change]

src/libstd/io/stdio.rs

@@ -176,8 +176,8 @@ pub fn set_stdout(stdout: Box<Writer + Send>) -> Option<Box<Writer + Send>> {
 /// Resets the task-local stderr handle to the specified writer
 ///
 /// This will replace the current task's stderr handle, returning the old
-/// handle. Currently, the stderr handle is used for printing failure messages
-/// during task failure.
+/// handle. Currently, the stderr handle is used for printing panic messages
+/// during task panic.
 ///
 /// Note that this does not need to be called for all new tasks; the default
 /// output handle is to the process's stderr stream.
@@ -212,7 +212,7 @@ fn with_task_stdout(f: |&mut Writer| -> IoResult<()>) {
     };
     match result {
         Ok(()) => {}
-        Err(e) => fail!("failed printing to stdout: {}", e),
+        Err(e) => panic!("failed printing to stdout: {}", e),
     }
 }
 
@@ -415,7 +415,7 @@ mod tests {
         let (mut r, w) = (ChanReader::new(rx), ChanWriter::new(tx));
         spawn(proc() {
             ::realstd::io::stdio::set_stderr(box w);
-            fail!("my special message");
+            panic!("my special message");
         });
         let s = r.read_to_string().unwrap();
         assert!(s.as_slice().contains("my special message"));