`traverseWithIndex_` for `Seq`

[sjakobi]

…would be nice to have.

dhall previously used Data.Foldable.sequenceA_ (Data.Sequence.mapWithIndex f xs), but this is not optimal because it the result of mapWithIndex is materialized before being reduced by sequenceA_.

dhall-lang/dhall-haskell#2423 switches dhall to use itraverse_ from indexed-traversable which has a nice implementation in terms of Data.Sequence.foldMapWithIndex.

[meooow25]

See also: #422 (comment)

[meooow25]

Using indexed-traversable for this should be fine (#422 (comment)).