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/libcore/lib.rs

@@ -71,4 +71,5 @@ pub mod option;
 pub mod raw;
 pub mod char;
 pub mod slice;
+pub mod str;
 pub mod tuple;