[modarith] migrate barrett/subifge to mod arith type

This would remove the mod arith op attributes tracking the modulus.

We realized this would be a nice "type" to have to describe the coefficient type of a ring. In the case of working with $$\mathbb{Z}_q$$, the coefficient type would be a modarith type like `modarith`. RNS types would be a list of modarith types.

See the type refactor here: https://github.com/google/heir/pull/1083