design: add 30333-smarter-scavenging
design: add 30333-smarter-scavenging

This change adds a design document/proposal for improving the
scavenging mechanisms in the Go runtime.

For golang/go#30333.

Change-Id: I2dc2cad8d5585347574f893ace970523e9889a72
Reviewed-on: https://go-review.googlesource.com/c/163217
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

golang/proposal,golang/proposal