Slide from left to right

Slide from left to right

Scalar multiplication goes from left to right (from MSB to
LSB). Computing the sliding windows used to go from *right to left*.

This direction mismatch forced us to keep all the signed digits in
memory, which currently incur a little over 500 bytes of stack overhead.
That overhead is avoidable. Avoiding it will allow Monocypher to fit in
smaller embedded devices.

Right now we just change the direction of the sliding. Interleaving will
come later.