Implement Agent

[Odomontois]

Implement new concurrent datatype Agent mostly like RefM from ZIO and looking like Ref + Semaphore (Atom + Gatekeeper) for everything else

It should have at least following methods

trait Agent[F[_], A]{
  // return current value, this will never block
  def get: F[A]
  // update value with effectful transformation, wait for the new result
  def updateM(f: A => F[A]): F[A]
  // enqueue transformation, return immediately
  def fireUpdateM(f: A => F[A]): F[Unit]
  // modify value with effectful transformation, wait for the new result
  def modifyM[B](f: A => F[(A, B)]): F[B]
}

[susliko]

Working on this

[susliko]

PR
#107

[Odomontois]

Closing for now, since simple version has been delivered.
We'll wait for your more advanced implementation @susliko