Const generic equivalent of `Curve::batch_normalize`

[tarcieri]

batch_normalize currently documents this caveat:

This function will panic if p.len() != q.len().

However, with const generics you can construct an output array that matches the input size with compile-time assurances:

fn batch_normalize_array<const N: usize>(points: &[Self, N]) -> [Self::AffineRepr; N] { ... }

This issue is to suggest an addition, rather than a replacement, specifically for use cases where the number of points is fixed at compile-time.

Note that this approach is also useful for stack allocating all intermediate values (since you can size the arrays by N), allowing for efficient implementations on heapless targets.

[tarcieri]

We ended up solving this with a BatchNormalize trait which is generic around its input type and therefore supports overlapping impls.

This allows the trait to be impl'd for [ProjectivePoint] and [ProjectivePoint; N] as inputs, where the former is gated on the alloc feature, and the latter supporting heapless usage, with the const generics completely disentangled from the trait definition: https://docs.rs/primeorder/0.13.6/primeorder/struct.ProjectivePoint.html#impl-BatchNormalize%3C%5BProjectivePoint%3CC%3E%5D%3E-for-ProjectivePoint%3CC%3E