proposal: design document for redesigned code coverage

Detailed design document for proposal golang/go#51430, revamping
code coverage testing.

Updates golang/go#51430.

Change-Id: I692c6c6f977b9e4e14a6c4cc5fbd086d53021c27
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/388857
Trust: Than McIntosh <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

Author: thanm