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/chain.rs

@@ -1,8 +1,6 @@
-use crate::buf::IntoIter;
+use crate::buf::{IntoIter, UninitSlice};
 use crate::{Buf, BufMut};
 
-use core::mem::MaybeUninit;
-
 #[cfg(feature = "std")]
 use std::io::IoSlice;
 
@@ -183,7 +181,7 @@ where
         self.a.remaining_mut() + self.b.remaining_mut()
     }
 
-    fn bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
+    fn bytes_mut(&mut self) -> &mut UninitSlice {
         if self.a.has_remaining_mut() {
             self.a.bytes_mut()
         } else {