The low-level `Curve25519.fromEdwards25519()` function assumed
that the X/Y coordinates were not scaled (Z=1).

But this is not guaranteed to be the case.

In most real-world applications, the coordinates are freshly decoded,
either directly or via the `X25519.fromEd25519()` function, so this
is not an issue.

However, since we offer the ability to do that conversion after
arbitrary computations, the assertion was not correct.