core: Remove generics from Option::expect

The prospects of a generic failure function such as this existing in libcore are
bleak, due to monomorphization not working across the crate boundary, and
allocation into a ~Any is not allowed in libcore.

The argument to expect() is now &str instead of <M: Send + Any>

[breaking-change]

Author: alexcrichton

src/libcore/option.rs

@@ -138,11 +138,9 @@
 //! }
 //! ```
 
-use any::Any;
 use cmp::{Eq, TotalEq, TotalOrd};
 use default::Default;
 use iter::{Iterator, DoubleEndedIterator, FromIterator, ExactSize};
-use kinds::Send;
 use mem;
 use slice;
 
@@ -238,7 +236,7 @@ impl<T> Option<T> {
     ///
     /// Fails if the value is a `None` with a custom failure message provided by `msg`.
     #[inline]
-    pub fn expect<M: Any + Send>(self, msg: M) -> T {
+    pub fn expect(self, msg: &str) -> T {
         match self {
             Some(val) => val,
             None => fail!(msg),