Expose STARK inner polynomials better

[hratoanina]

For a given STARK, the way the trace, auxiliary and quotient polynomials are stored is obscure. In particular, the order of auxiliary polynomials in the auxiliary Merkle tree leaves is:

• Lookup columns
  • Lookup 0
    • Challenge 0
    • Challenge 1
    • ...
  • Lookup 1
  • ...
• CTL columns
  • CTL helper columns
    • CTL 0
      • Challenge 0
      • Challenge 1
      • ...
    • CTL 1
    • ...
  • CTL Z columns
    • CTL 0
      • Challenge 0
      • Challenge 1
      • ...
    • CTL 1
    • ...

This order is only implied by the relevant helper methods, and it makes it messy to e.g. access only the CTL Z columns. We should at least have this order clearly mentioned somewhere, or an even cleaner way would be to maintain a struct per STARK with the relevant ranges, and implement proper getters out of them.