import { ed } from "https://deno.land/x/earthstar@v10.0.0/deps.ts";
const { Point } = ed;
Converts to Montgomery; aka x coordinate of curve25519. We don't have fromX25519, because we don't know sign.
u, v: curve25519 coordinates
x, y: ed25519 coordinates
(u, v) = ((1+y)/(1-y), sqrt(-486664)*u/x)
(x, y) = (sqrt(-486664)*u/v, (u-1)/(u+1))