Allow to copy an existing mock

[polarene]

Prerequisites

Please answer the following questions for yourself before submitting an issue.

• I am running the latest version
• I checked the documentation and found no answer
• I checked to make sure that this issue has not already been filed

Expected Behavior

I've encountered a scenario where I needed more than one mock of the same type but with a subset of different stubbings. Initializing each mock from scratch leads to duplication, which can only be avoided writing a custom function.
I'd like to have a copy factory function so I could write:

val first = mockk<Car> {
  // base stubbings
  every { drive(Direction.NORTH) } returns Outcome.OK
  every { door(DoorType.FRONT_LEFT).windowState() } returns WindowState.UP
}
val second = mockk(first) {
  // redefine a subset of stubbings
  every { drive(Direction.SOUTH) } returns Outcome.ERROR
}

second will be a new mock instance having all the configuration copied over, and its stubbings don't affect first.

Current Behavior

The library currently doesn't support this feature.

[polarene]

I've started working on this feature but I'm still studying how to clone the internal data structures.