The "Parsed JSON number <1000000000000000070> does not fit in Int64." error occurs when running the following code on iOS 10.
It seems that on iOS 10, for some big numbers that fit in an Int64, JSONSerialization creates not a normal NSNumber but an instance of NSDecimalNumber.
On iOS 11, a big number like the one above becomes an NSNumber so everything works fine.
iOS 10 probably can't be fixed but it seems to me that changing the code at https://github.com/apple/swift/blob/a62647f0685c3c832756d9f635cfe527d45037de/stdlib/public/SDK/Foundation/JSONEncoder.swift#L2100-L2103 to the code below would fix the problem.