Improve verification key initialization from fields elements

[vezenovm]

As per these comments here: #383 (comment)

The initialization of a verification key from a vector of field elements could be improved. Currently the function uses magic numbers and makes us dependent on order. We should look into returning some sort of initializer struct as mentioned in the comments linked.

[ledwards2225]

This is handled more robustly for Honk than it was for Plonk