Dynamic should default to vectors as the collection tag, instead of lists

[kubukoz]

Currently, if you load a list shape using DynamicSchemaIndex, it'll always have CollectionTag.ListTag. It should be changed to VectorTag, as vectors may be a more sensible default collection type for dynamic usecases (as you won't be pattern matching on these lists, for example).

I think this can go in a patch release of <current major series, at the time of writing this is 0.18>.