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/libcore/lib.rs

@@ -40,8 +40,8 @@
 //!
 //! * `rust_begin_unwind` - This function takes three arguments, a
 //!   `&fmt::Arguments`, a `&str`, and a `uint`. These three arguments dictate
-//!   the failure message, the file at which failure was invoked, and the line.
-//!   It is up to consumers of this core library to define this failure
+//!   the panic message, the file at which panic was invoked, and the line.
+//!   It is up to consumers of this core library to define this panic
 //!   function; it is only required to never return.
 
 // Since libcore defines many fundamental lang items, all tests live in a
@@ -111,7 +111,7 @@ pub mod atomic;
 pub mod bool;
 pub mod cell;
 pub mod char;
-pub mod failure;
+pub mod panicking;
 pub mod finally;
 pub mod iter;
 pub mod option;
@@ -129,7 +129,7 @@ pub mod fmt;
 
 #[doc(hidden)]
 mod core {
-    pub use failure;
+    pub use panicking;
 }
 
 #[doc(hidden)]