Fix bug in ArrayVec::extend_from_slice

Currently the source slice must be the exact length to fill the array to
max capacity, this is an unnecessary restriction since an `ArrayVec` is
a variable sized data structure.

Set the destination slice to be the same length as the source
slice (still maintain capacity checks).

internals/src/array_vec.rs

@@ -78,7 +78,7 @@ mod safety_boundary {
             assert!(new_len <= CAP, "buffer overflow");
             // SAFETY: MaybeUninit<T> has the same layout as T
             let slice = unsafe { &*(slice as *const _ as *const [MaybeUninit<T>]) };
-            self.data[self.len..].copy_from_slice(slice);
+            self.data[self.len..new_len].copy_from_slice(slice);
             self.len = new_len;
         }
     }