Add non mut spare_capacity().

Author: jaamesd

src/bytes_mut.rs

@@ -1117,6 +1117,17 @@ impl BytesMut {
             slice::from_raw_parts_mut(ptr.cast(), len)
         }
     }
+
+    /// Returns the remaining spare capacity of the buffer.
+    #[inline]
+    pub fn spare_capacity(&self) -> &[MaybeUninit<u8>] {
+        unsafe {
+            let ptr = self.ptr.as_ptr().add(self.len);
+            let len = self.cap - self.len;
+
+            slice::from_raw_parts(ptr.cast(), len)
+        }
+    }
 }
 
 impl Drop for BytesMut {