Const generics

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There are a few issues with const generics at the moment:

1. FromIterator is not implemented for [T; N]. So there are no ways to collect an iterator without unsafe or default (which is harder to implement in this case). More on that: Provide a means of turning iterators into fixed-size arrays rust-lang/rust#81615
2. Serde does not support [T; N] because it breaks backwards compatibility (Const generics support  serde-rs/serde#1937).
3. Because of the issues above it's not possible to get rid of SizedVec completely, unless we:
   1. Update https://github.com/matter-labs-archive/blake2_rfc_bellman_edition to arrayvec 0.7.
   2. Then we can use ArrayVec instead of SizedVec, since it's already a dependency.