design: add mid-stack inlining design doc
design: add mid-stack inlining design doc

For golang/go#19348.

Change-Id: Ibf3e3817b35226a33d961e76fedb924e15e37069
Reviewed-on: https://go-review.googlesource.com/38090
Run-TryBot: David Lazar <859c8a6d485c34c743019c2e216e6c66f2ebb470@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

golang/proposal,golang/proposal