fix gen'd struct field lookups to keep ptrness.

Before this diff, when getting a field back out, if it was nullable or
optional, you'd get one more level of pointer than you started with.
Oddly, this still compiled.  Tests sensibly rejected it, though.
Signed-off-by: Eric Myhre <hash@exultant.us>

schema/gen/go/_test/wow_test.go

@@ -62,6 +62,9 @@ func TestStructBuilder(t *testing.T) {
 			Wish(t, err, ShouldEqual, nil)
 			Wish(t, n.ReprKind(), ShouldEqual, ipld.ReprKind_Map)
 			Wish(t, plz(n.LookupString("f1")), ShouldEqual, plz(String__NodeBuilder{}.CreateString("a")))
+			Wish(t, plz(n.LookupString("f2")), ShouldEqual, plz(String__NodeBuilder{}.CreateString("b")))
+			Wish(t, plz(n.LookupString("f3")), ShouldEqual, plz(String__NodeBuilder{}.CreateString("c")))
+			Wish(t, plz(n.LookupString("f4")), ShouldEqual, plz(String__NodeBuilder{}.CreateString("d")))
 		})
 	})
 }

schema/gen/go/genKindStruct.go

@@ -79,7 +79,11 @@ func (gk generateKindStruct) EmitNodeMethodLookupString(w io.Writer) {
 					return ipld.Null, nil
 				}
 				{{- end}}
+				{{- if or $field.IsOptional $field.IsNullable }}
+				return *x.{{ $field.Name }}, nil
+				{{- else}}
 				return x.{{ $field.Name }}, nil
+				{{- end}}
 			{{- end}}
 			default:
 				return nil, typed.ErrNoSuchField{Type: nil /*TODO*/, FieldName: key}