This should contain generic implementations of scalar multiplication algorithms
that can be used with multiple backends. The goal is to move the existing
scalar multiplication code into this submodule, then call it from the
user-facing API. This can also contain code for things we can't do now, like
multiscalar multiplication with precomputation.
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.
