spec: add section on the structure of interfaces
spec: add section on the structure of interfaces

This change introduces the notion of a structural interface
and its corresponding structural type.

Change-Id: Ib5442dfd04cb5950b4467428cae51849f8922272
Reviewed-on: https://go-review.googlesource.com/c/go/+/365474
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

CAFxX/go,nawawi/go,golang/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,golang/go