[dev.typeparams] cmd/compile/internal/types2: implement type terms

Type terms will be used to represent a type set as a list
of type terms. Eventually, a type term may also include
a method set.

Groundwork for the implementation of lazily computed
type sets for union expressions.

Change-Id: Ic88750af21f697ce0b52a2259eff40bee115964c
Reviewed-on: https://go-review.googlesource.com/c/go/+/338049


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