ModArith: Support RNS type in ops

[znssong]

Support RNS type in ModArith operations. We start from supporting mod_arith.add operation.

[j2kun]

This works well with the verifier, but in the future it might be useful to add a type constraint to allow you to express RNS types for which all inner types match a given constraint. Then you could do something like RnsOf<ModArith_ModArithType>

[ZenithalHourlyRate]

Just a comment for future PR:

verifyModArithType could be type verifier by using let genVerifyDecl = 1; in ModArithType.td, instead of being called by each op verifier. By then we may get rid of all these modarith/rns verifier for ops and all these op type constraints are maintained in tablegen.

This is kind of legacy code when the ModArithType was introduced (a8eb769), and the verifyModArithType was verifyModArithOpMod, and it was not migrated.