Would be good to have the basepoint itself, but this doesn't seem possible: the
fields of an ExtendedPoint are not public, so we can't construct them outside
of curve.rs (by design, to prevent constructing invalid points). Maybe const
fn support could change this.
This was presumably added for use with formulas that use two multiplications by
small constants (i.e., 121665 and 121666) instead of one multiplication by a
large constant (i.e., -121665/121666), but we don't use those formulas.
