Make WreathProductElementList for Matrix Wreath Product faster.

Use `BlockMatrix` instead of computing a product and embeddings.

At the moment we use `MatrixByBlockMatrix` to convert `BlockMatrix` to `IsOrdinaryMatrix`.
Without conversion we get errors.

lib/gprdmat.gi

@@ -495,7 +495,8 @@ function(G, list)
   local info;
 
   info := WreathProductInfo(G);
-  return Product(List([1 .. info.degI + 1], i -> list[i] ^ Embedding(G, i)));
+  # TODO: Remove `MatrixByBlockMatrix` when `BlockMatrix` supports the MatObj interface.
+  return MatrixByBlockMatrix(BlockMatrix(List([1 .. info.degI], i -> [i, i ^ list[info.degI + 1], list[i]]), info.degI, info.degI));
 end);
 
 # tensor wreath -- dimension d^e This is not a faithful representation of