Change Scene's ReturnValue associetedType

[crexista]

Detail
This commit change Scene's ReturnValue gained through rewind method was Optional to Non Optional.
Because, it is inconvenient because it is necessary to check nil when receiving RetuneValue.

Breaking Change
initialize TransitionRequest have to change this.

• before

let request = HogeRequest() { ret in // result is optional
   guard let result = result else { return }
   // process using result
}

• after

let request = HogeRequest() { ret in // result is non-optional
   // process using result
}