StrictSerializedTransformable - SerializedTransformable that doesn't result in Optional types

[hex2f]

This is probably far from the best way to do something like this, may even be possible in the original package with the methods provided, but I couldn't find a way to do it. Just thought i'd share this as a PR in case anyone finds it useful and wants to refine the concept to be merged into the main branch. If not, feel free to close c:

Example of StrictSerializedTransformable

• Like SerializedTransformable but for non-optional types

 class IntToStringTransformer: StrictTransformable {
    static func transformFromJSON(value: Int) -> String {
        return String(value)
    }
    
    static func transformToJSON(value: String) -> Int {
        return Int(value) ?? 0
    }
 }
 // Usage of `StrictSerializedTransformable`
 struct User: Serializable {
     @StrictSerializedTransformable<IntToStringTransformer>
     var id: String
 }