[dev.typeparams] cmd/compile/internal/types: review of objset.go

Except for the package name, this file is unchanged from the go/types version.

f=objset.go; diff $f ../../../../go/types/$f
11c11
< package types2
---
> package types

Change-Id: I5a03b08ec006d87cb31139f708d844fcfddbbb56
Reviewed-on: https://go-review.googlesource.com/c/go/+/265699


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>