Expose mechanism for viewing system fields

Hey Gui,

It would be great to have access to the system fields typed data of the CKRecord to provide the capability to the following workflows:
1. Inspect recordChangeTag to detect conflicts before executing an CKModifyRecordsOperation
2. Inspect the creatorUserRecordID for presentation to the user or logic purposes
Example: owner can update, but enforce this client side--not by using security role
3. Inspect modificationDate for display or logic purposes
4. Etc for other system fields

Do you think this would compliment your library? I assume the above use cases would be commonly desired. Here is some quick and dirty on what I was thinking.

```
public struct CloudKitRecordSystemFields {

private let metaRecord: CKRecord

public var recordID: CKRecord.ID {
metaRecord.recordID
}

public var recordType: CKRecord.RecordType {
metaRecord.recordType
}

public var creationDate: Date? {
metaRecord.creationDate
}

public var creatorUserRecordID: CKRecord.ID? {
metaRecord.creatorUserRecordID
}

public var modificationDate: Date? {
metaRecord.modificationDate
}

public var lastModifiedUserRecordID: CKRecord.ID? {
metaRecord.lastModifiedUserRecordID
}

public var recordChangeTag: String? {
metaRecord.recordChangeTag
}

public init(record: CKRecord) {
self.metaRecord = record
}
}

extension CloudKitRecordSystemFields {

// Discussion: Static factory, convenience initializers, or both?

public init(coder: NSCoder) {
guard let metaRecord = CKRecord(coder: coder) else {
// NOTE: Might have to use generics to infer recordType if coder initializer of the metaRecord fails if returning a default value instead of throw or nil
fatalError()
}

self.metaRecord = metaRecord
}

public static from(coder: NSCoder) -> CloudKitRecordSystemFields {
// TODO: Implement this signature
fatalError()
}

public init(data: Data) {
// TODO: Implement this signature
fatalError()
}

public static func from(data: Data) -> CloudKitRecordSystemFields {
// TODO: Implement this signature
fatalError()
}
}
```