spec: generalize method sets and interface types to type sets

This is the first of several CLs that update the existing
Go 1.17 spec for type parameters.

This CL updates the section on method sets and interface types.
It also adds "any", "comparable" to the list of predeclared
identifiers.

Change-Id: I0ce25dc02791c33150c0d949528512610faf3eab
Reviewed-on: https://go-review.googlesource.com/c/go/+/362999


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>