core: Inherit non-allocating slice functionality

This commit adds a new trait, MutableVectorAllocating, which represents
functions on vectors which can allocate.

This is another extension trait to slices which should be removed once a lang
item exists for the ~ allocation.

src/libcore/lib.rs

@@ -69,4 +69,5 @@ pub mod iter;
 pub mod option;
 pub mod raw;
 pub mod char;
+pub mod slice;
 pub mod tuple;