Fix semantic bug in TypeUnion: lookup key is not always the member type's name!

While fixing this it occurs to me that we could in theory also support
more than one name alias that will be mapped onto the same concrete type.
...
But Let's Not with that.  It would increase the code complexity and more
importantly, *it would be lossy to do so*.  So let's not open that can
of worms: straying into lossiness territory is far worse than whatever
other hypothetical problems one might imagine trying to solve.
Signed-off-by: Eric Myhre <hash@exultant.us>

typed/system/typeMethods.go

@@ -99,10 +99,17 @@ func (t TypeList) ValueIsNullable() bool {
 }
 
 // UnionMembers returns a set of all the types that can inhabit this Union.
-func (t TypeUnion) UnionMembers() map[TypeName]Type {
-	m := make(map[TypeName]Type, len(t.values))
-	for k, v := range t.values {
-		m[k] = v
+func (t TypeUnion) UnionMembers() map[Type]struct{} {
+	m := make(map[Type]struct{}, len(t.values)+len(t.valuesKinded))
+	switch t.style {
+	case UnionStyle_Kinded:
+		for _, v := range t.valuesKinded {
+			m[v] = struct{}{}
+		}
+	default:
+		for _, v := range t.values {
+			m[v] = struct{}{}
+		}
 	}
 	return m
 }

typed/system/types.go

@@ -125,7 +125,7 @@ type TypeUnion struct {
 	anyType
 	style        UnionStyle
 	valuesKinded map[ipld.ReprKind]Type // for Style==Kinded
-	values       map[TypeName]Type      // for Style!=Kinded (always defined, just not used in Kinded lookups)
+	values       map[string]Type        // for Style!=Kinded (note, key is freetext, not necessarily TypeName of the value)
 	typeHintKey  string                 // for Style==Envelope|Inline
 	contentKey   string                 // for Style==Envelope
 }