Remove BufMut::bytes_vectored_mut() (#430)

There are issues with regard to uninitialized memory. We are avoiding
stabilizing this function for now.

src/buf/mod.rs

@@ -24,7 +24,5 @@ mod vec_deque;
 
 pub use self::buf_impl::Buf;
 pub use self::buf_mut::BufMut;
-#[cfg(feature = "std")]
-pub use self::buf_mut::IoSliceMut;
 pub use self::ext::{BufExt, BufMutExt};
 pub use self::iter::IntoIter;