design: type parameters: various cleanups
design: type parameters: various cleanups

Also drop the idea that we implicitly pass a type parameter to a
parameterized type constraint. We can use an interface type literal to
get a similar effect.

Change-Id: Iebabb9669d5fcd88a79e0c5388f74c4c1d9c0325
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/251137
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/proposal,golang/proposal