QrCode's Index trait is indexing (col, row) instead of (row, col)

[adityapk00]

The Index<> for QrCode implementation here seems to have interchanged the row and column, indexing into the array the wrong way.

The other functions (like to_colors()) is returning row-major order, while the Index is the only one returning column-major.

The current code is:

let index = y * self.width + x;

It should probably be

let index = x * self.width + y;