Remove the Vec and String reexports at the root of the alloc crate

… since `std` has no corresponding reexports.

Use `alloc::vec::Vec` and `alloc::string::String` instead.

src/liballoc/lib.rs

@@ -179,8 +179,3 @@ pub mod vec;
 mod std {
     pub use core::ops;      // RangeFull
 }
-
-#[doc(no_inline)]
-pub use string::String;
-#[doc(no_inline)]
-pub use vec::Vec;