core: Inherit possible string functionality

This moves as much allocation as possible from teh std::str module into
core::str. This includes essentially all non-allocating functionality, mostly
iterators and slicing and such.

This primarily splits the Str trait into only having the as_slice() method,
adding a new StrAllocating trait to std::str which contains the relevant new
allocation methods. This is a breaking change if any of the methods of "trait
Str" were overriden. The old functionality can be restored by implementing both
the Str and StrAllocating traits.

[breaking-change]

src/libstd/task.rs

@@ -49,7 +49,7 @@ use str::{Str, SendStr, IntoMaybeOwned};
 
 #[cfg(test)] use any::{AnyOwnExt, AnyRefExt};
 #[cfg(test)] use result;
-#[cfg(test)] use str::StrSlice;
+#[cfg(test)] use str::StrAllocating;
 
 /// Indicates the manner in which a task exited.
 ///