design/60773-execution-tracer-overhaul.md: add design

For golang/go#60773.

Change-Id: I0f35c07941b5ebafc4e57db037a4a92092d6c7b0
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/503038
Reviewed-by: Michael Pratt <[email protected]>

Author: mknyszek