1) Elligator inversion fails on 0.  Also there may be corner cases
here which ought to be probed but are a pain, such as sqrt(id/(1-d))
and similar.

2) Elligator doesn't return the right hint, because I haven't coded
the rotation hints.  Probable solution: make Elligator not return a
hint, because there's no realistic scenario where it's useful anyway.
Alternative possible solution: can compute the right hint, but why
bother?

3) Elligator inversion doesn't set the high bit of the buffer at
random, because 2^255-19 isn't close to 2^256.  Possible solution:
preserve the high bit(s) of the buffer?

4) Elligator doesn't map [1] to the identity, I think.

5) Not enough corner case testing.

6) Probably some other non-Elligator problems

I'm kind of torn about this change, because it adds a bunch of
fairly complex code that's only needed for esoteric use cases,
and it makes Elligator more complex, if mostly only for testing
purposes.  Basically, this is because Elligator is approximately
~8-to-1 when its domain is 56 bytes: 2 because it's [0..p+small]
instead of [0..(p-1)/2], and 4 for cofactor removal.  So when you
call the inverse on a point, you need to say which inverse you want,
i.e. a "hint".

Of course, the inverse fails with probability 1/2.

To make round-tripping a possibility (I'm not sure why you'd need this),
the Elligator functions now return an unsigned char hint.  This means
that you can call Elligator, and then invert it with the hint you gave,
and get the same buffer back out.  This adds a bunch of complexity to
Elligator, which didn't previously need to compute hints.  The hinting is
reasonably well tested, but it is known not to work for inputs which are
very "large", i.e. end ~28 0xFF's (FIXME.  Or roll back hinting...).

There's also a significant chance that I'll revise the hinting mechanism.

Create functions:
    decaf_448_invert_elligator_nonuniform
    decaf_448_invert_elligator_uniform
    decaf::Ed448::Point::invert_elligator
    decaf::Ed448::Point::steg_encode

for inverting Elligator.  This last one encodes to Point::STEG_BYTES = 64
bytes in a way which is supposed to be indistinguishable from random, so
long as your point is random on the curve.

Inverting Elligator costs about 2 square roots for nonuniform.  For
uniform, it's just Elligator -> diff -> invert, so it's 3 square roots.
Stegging fails about half the time, and so costs about twice that, but
the benchmark underreports it because it ignores outliers.

The code is tested, but I haven't checked over the indistinguishability
from random (I've only proved it correct...).  There could well be a way
to break the steg even without taking advantage of "very large" inputs
or similar.