Circuit exporter

[vasinov]

Users should be able to export their circuits either with a custom QuantumContext implementation or a new abstraction (e.g., QuantumExporter).

If we decide to go down the path of extending QuantumContext then run's signature should change to something like:

sealed trait Result
trait State extends Result
trait ExportedCircuit extends Result

case class QuantumContext[T <: Result] {
  run(): T
 // ...
}

Might be a better idea to just have QuantumExporter, since it's enough of a separate use case.

Initial exporter implementations should have Quil and OpenQASM support.