[#9] Add support for atomic updates with Mutekt models

[PatilShreyas]

Summary

• Added support for atomic updates for multiple state fields of a Mutekt model.
  Fixes: How do you do atomic update with this ? #9
  As discussed: Adding support for atomic updates for two or more state field updates #12

For example: If the following is a model

@GenerateMutableModel
interface NotesState {
  val loading: Boolean
  val notes: List<String>
  val error: String?
}

Then, atomic update will be possible with update{} method:

val notesState = MutableNotesState(...)

notesState.update {
  loading = false
  notes = listOf("Lorem Ipsum")
}

Here, on mutating loading and notes field of a state, only a a single value will be emitted as a state without affecting the current coroutine dispatcher.

• Added interfaces MutektState and MutektMutableState. Mutable model implements it.