RS encoding and decoding for shards

[snjax]

Represent data as a table with N=2^n rows and M=2^m columns.

Each column corresponds to polynomial coefficients, and each shard corresponds to a point at m31 circle coset with order N*B, where B is the blowup factor.

• Implement computation of shards rows using columnwise_dot_product (see example )
• Implement recovery from any N elements from the same coset (N-ordered, there are B such cosets)
• Implement recovery from any arbitrary N elements, using an inefficient O(N^2) algorithm