Add an API which can download, decode, and decrypt shares given a CHK read capability and some servers

Something like:

import Data.Map.Strict (Map)
import Tahoe.CHK.Server (StorageServerAnnouncement)
import Tahoe.CHK.Capability (Reader)
import Data.ByteString.Lazy (ByteString)

data HighLevelError

download :: Map StorageServerIdentifier StorageServerAnnouncement -> Reader -> IO (Either HighLevelError ByteString)