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 <[email protected]>

Author: mknyszek