`bearriver`: Offer all definitions from `FRP.Yampa.Task`

[ivanperez-keera]

The module FRP.Yampa.Task is being offered by Yampa, but not bearriver.

This makes the interface offered by bearriver not compliant with Yampa, therefore not delivering on the promise of being API-compatible.

This is a subissue of #40. Because #40 is a big issue, it's good to split it in several smaller ones.