Change module TaskSeq -> type TaskSeq for extension members

[abelbraaksma]

This is in preparation of #184 and #133.

While this is technically binary incompatible, for all intents and purposes, existing code would just remain working "as is".

Note that I deliberately chose to stick with the pascalCase convention over the conventions used by F# on the Async methods. That's in part because we already started along that route (we would like to remain backwards compatible), and because it resembles the look and feel of module functions of Seq.

[abelbraaksma]

Time to merge this one to open the door for the coming-up stuff :).