Make DAVerifier functions external instead of public

[adlerjohn]

verifyRowRootToDataRootTupleRoot and verifyMultiRowRootsToDataRootTupleRoot are public, when they can be external.

[adlerjohn]

Actually, this brings up a question of if the library should only have internal functions or some external functions. If the former, then it can be compiled inline and reduce gas costs in some cases at higher deploy cost.

[rach-id]

I think internal makes more sense since rollups will be using their contracts for verification instead of calling external ones. So it makes sense to make them internal

[Ferret-san]

From talking to rollup teams, they generally prefer internal libraries when there's contract space for it. And usually there's space for it in the contracts where the libraries would be used.