[RFC] Improving compact x86-64 compact unwind descriptors

Apologies for the readability/formatting of the document. It started
out as Word doc. I've put the PDF in my DropBox

https://www.dropbox.com/s/hyvit0tnfao19v3/Asynchronous%20Compact%20Unwind%20Descriptors%202018-01-25.pdf?dl=0

John