Change save format to Quetzal

[SloeComputers]

Current save format is little more than a raw context, stack, and memory dump. Switch to the more efficient and interchangeable with other Z engine players Quetzal format as described in the Z-Machine Standards Document.

[SloeComputers]

Save format is now Quetzal. But the stack layout inside the Quetzal file is different to other engines (frotz) so not compatible. Need to either change the stack layout or translate the format on save and restore.