design: expand non-cooperative preemption
design: expand non-cooperative preemption

Add more discussion of unsafe-points. Describe the stack/register map
encoding. Add another alternative approach.

Change-Id: I0315ee7d3ccd9e511243f16d58d3ca9802e9ea77
Reviewed-on: https://go-review.googlesource.com/122795
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/proposal,golang/proposal