This change provides a common convention for using allocator-dependent
features with:
#![cfg(feature = "alloc")]
When available, `Vec` is imported consistently as `prelude::Vec`, which
means modules that need access to `Vec` can simply do:
use prelude::*;
and if an allocator is available, `Vec` will be in the crate prelude.
This allows all `alloc` vs `std` gating to be handled in `lib.rs`,
`build.rs`, and `prelude.rs` so the rest of the codebase doesn't have to
do any gating whatsoever.
This changes the primary function for the `VartimeMultiscalarMul` trait
to an `optional_multiscalar_mul` trait that accepts
`Option<Self::Point>` (and returns `None` if any input points are
`None`).
The existing `vartime_multiscalar_mul` is changed to be a wrapper around
this function to avoid code duplication. This may result in an
extra copy of each input point, but that cost is probably not
significant compared to the cost of the multiscalar multiplication.
The motivation is to allow performing multiscalar multiplications with
inline decompression. Currently, API consumers have to allocate
temporary buffers for all of their points, decompress into those
buffers, then pass (iterators over) those buffers into the multiscalar
multiplication code, which then creates new buffers for lookup tables.
