Switch BufMut::bytes_mut to&mut UninitSlice (#433)

The way BufMut uses MaybeUninit can lead to unsoundness. This replaces
MaybeUnit with a type owned by bytes so we can ensure the usage patterns
are sound.

Refs: #328

src/buf/mod.rs

@@ -24,6 +24,7 @@ mod limit;
 #[cfg(feature = "std")]
 mod reader;
 mod take;
+mod uninit_slice;
 mod vec_deque;
 #[cfg(feature = "std")]
 mod writer;
@@ -34,6 +35,7 @@ pub use self::chain::Chain;
 pub use self::iter::IntoIter;
 pub use self::limit::Limit;
 pub use self::take::Take;
+pub use self::uninit_slice::UninitSlice;
 
 #[cfg(feature = "std")]
 pub use self::{reader::Reader, writer::Writer};