Arrabbiata: add selectors in IndexedRelation

[dannywillems]

The task consists of listing the selectors, commit to them, and absorb them in the transcript at setup time.
In addition to that, the witness environment should not keep track of it in their internal structure.

This issue would be considered close when the following FIXME will be addressed:

• https://github.com/o1-labs/proof-systems/blob/master/arrabbiata/src/setup.rs#L92
• https://github.com/o1-labs/proof-systems/blob/master/arrabbiata/src/setup.rs#L190
• https://github.com/o1-labs/proof-systems/blob/master/arrabbiata/src/witness.rs#L849

In addition to that, the selectors field here should be removed.

The PR introducing the cross-terms computations already has some commits that can be considered to be used.