Add a safe way to create UninitSlice from slices (#598)

Introduce UninitSlice::from_slice and UninitSlice::from_uninit_slice
methods which safely create Uninit slice from provided slice of maybe
uninitialised or initialised memory.

In addition, add `From<&mut [T]>` implementations (for `T=u8` and
`T=MaybeUninit<u8>`) which do conversion from slice to UninitSlice.

Closes: #552

src/bytes_mut.rs

@@ -1102,7 +1102,7 @@ unsafe impl BufMut for BytesMut {
         if self.capacity() == self.len() {
             self.reserve(64);
         }
-        UninitSlice::from_slice(self.spare_capacity_mut())
+        self.spare_capacity_mut().into()
     }
 
     // Specialize these methods so they can skip checking `remaining_mut`