Fix Traverse for Async seqs

[gusty]

The problem with the current implementation is that it ignores overloads defined in the custom IEnumerable.

Moreover, in these cases the resulting type is wrong:

> let b = traverse async.Return (MyCustomIEnumerable.ofSeq (Seq.initInfinite id))  ;;
val b: Async<int seq>   ===> it should be Async<int MyCustomIEnumerable>

[gusty]

This was reproduced here 917b669 and fixed at ba1d4ff