Small Redesign for `trait DataSource`

[diesalbla]

Consider if it would be convenient to modify the DataSource trait, by moving the F[_] parameter to the trait, and the type-class bounds into protected methods of the trait. I.e., that would be replacing the following:

trait DataSource[I, A] {
  def fetch[F[_] : ConcurrentEffect : Par](id: I): F[Option[A]]

by the following:

trait DataSource[F[_], I, A] {
  protected implicit def conc: ConcurrentEffect[F]
  protected implicit def par: Par[F] 
  def fetch(id: I): F[Option[A]]