The public-facing types with arithmetic operations are:
- `Scalar`s
- `ExtendedPoint`s
- `RistrettoPoint`s
For these types we define operators with all combinations of borrowed and
non-borrowed inputs, to avoid forcing API consumers to write extra ampersands.
Since all of the operations involved with these types are expensive relative to
the cost of an unnecessary copy, this isn't a big deal.
The `MontgomeryPoint` struct isn't included in the above because it's only
useful for scalar multiplication.
This commit is based on work by @UnlawfulMonad.
Originally this was for hygiene, so that we could erase points from
heap-allocated memory in multiscalar_mult, but it ends up providing a cleaner
API for scalar multiplication.
It's kept inside curve_models for now, but it could go somewhere else if that's a better place.
