Don't use the Montgomery ladder for EdDSA

Don't use the Montgomery ladder for EdDSA

Again.

There are two reasons: the first is that there are 2 special cases where
the ladder or conversion gives the wrong results (the ladder is correct,
it was just hijacked for another purpose).

The second reason is, fixed based scalar multiplication can be made even
faster in Twisted Edwards space, by using a comb algorithm.  The current
patch uses a window, so it is slower.  It will get faster again.