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/dynamic_lib.rs

@@ -44,7 +44,7 @@ impl Drop for DynamicLibrary {
             }
         }) {
             Ok(()) => {},
-            Err(str) => fail!("{}", str)
+            Err(str) => panic!("{}", str)
         }
     }
 }
@@ -168,13 +168,13 @@ mod test {
         // statically linked in
         let none: Option<Path> = None; // appease the typechecker
         let libm = match DynamicLibrary::open(none) {
-            Err(error) => fail!("Could not load self as module: {}", error),
+            Err(error) => panic!("Could not load self as module: {}", error),
             Ok(libm) => libm
         };
 
         let cosine: extern fn(libc::c_double) -> libc::c_double = unsafe {
             match libm.symbol("cos") {
-                Err(error) => fail!("Could not load function cos: {}", error),
+                Err(error) => panic!("Could not load function cos: {}", error),
                 Ok(cosine) => mem::transmute::<*mut u8, _>(cosine)
             }
         };
@@ -183,7 +183,7 @@ mod test {
         let expected_result = 1.0;
         let result = cosine(argument);
         if result != expected_result {
-            fail!("cos({}) != {} but equaled {} instead", argument,
+            panic!("cos({}) != {} but equaled {} instead", argument,
                    expected_result, result)
         }
     }
@@ -199,7 +199,7 @@ mod test {
         let path = Path::new("/dev/null");
         match DynamicLibrary::open(Some(&path)) {
             Err(_) => {}
-            Ok(_) => fail!("Successfully opened the empty library.")
+            Ok(_) => panic!("Successfully opened the empty library.")
         }
     }
 }