cmd/compile: design doc explaining unified IR implementation

[mdempsky]

The Go 1.18 implementation of generics is documented at https://github.com/golang/proposal/blob/master/design/generics-implementation-dictionaries-go1.18.md. Unified IR implements substantially the same idea, but there are some notable distinctions that would be worth preparing an updated design document to explain the implementation in full.

[mknyszek]

Moving to Backlog since this doesn't seem like it needs to be tied to a Go release. Please comment if you disagree or would like it in a milestone for tracking. Thanks!