Most of the functions related to points on the Edwards25519 curve
check that input points are not in a small-order subgroup.
They don't check that points are on the prime-order subgroup,
because this is expensive, and not always necessary.
However, applications may require such a check in order to
ensure that a public key is valid, and that a secret key counterpart
exists.
Many functions in the public API of libsodium related to arithmetic
over Edwards25519 also do that check unconditionally. This is
expensive, but a good way to catch bugs in protocols and
implementations.
So, add a `rejectUnexpectedSubgroup()` function to achieve this.
The documentation on the edwards25519->curve25519 conversion
function was also updated, in order to explain how to match
libsodium's behavior if necessary.
We use an addition chain to multiply the point by the order of
the prime group.
An alternative we may implement later is Pornin's point halving
technique: https://eprint.iacr.org/2022/1164.pdf
332fbb4b02