[dev.typeparams] go/types: merge instance and Named to eliminate sanitization

Storing temporary syntactic information using an *instance type forces
us to be careful not to leak references to *instance in the checker
output. This is complex and error prone, as types are written in many
places during type checking.

Instead, temporarily pin the necessary syntactic information directly to
the Named type during the type checking pass. This allows us to avoid
having to sanitize references.

This includes a couple of small, unrelated changes that were made in the
process of debugging:
 - eliminate the expandf indirection: it is no longer necessary
 - include type parameters when printing objects

For #46151

Change-Id: I767e35b289f2fea512a168997af0f861cd242175
Reviewed-on: https://go-review.googlesource.com/c/go/+/335929


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>