Create summoners for specialized Readers

[Odomontois]

Make it possible to summon specific contextual instances via

HasContext[F, C]
HasLocal[F, C]    
HasContextRun[F, G, C]

[DoSOfRedRiver]

I currently using a solution which works like:

  trait Id
  trait Templates

  type HasTemplates[F[_]] = HasContext[F, Templates]
  type HasId[F[_]] = HasContext[F, Id]

  def example[F[_]: HasTemplates: HasId: Monad] = {
    for {
      id <- hasContext[HasId].get
      templates <- hasContext[HasTemplates].get
    } yield ()
  }

Not sure if it is good enough, though. What do you think?

[atapin]

@Odomontois should this issue be closed?