Move codegen demo output to ignored dir.

The underscore makes it ignored by the go tools;
the gitignore does, well, git.

At some point this should probably grow up to have more complete
tooling and test suites around it, but for early iteration,
I pretty much have one window open to the generated output file,
and I'm just running it to see if it compiles, and while this probably
won't last for long, it's enough to get exploratory work done.

typed/gen/_test/.gitignore

+/*

typed/gen/gen_test.go

@@ -9,7 +9,7 @@ import (
 )
 
 func TestNuevo(t *testing.T) {
-	os.Mkdir("test", 0755)
+	os.Mkdir("_test", 0755)
 	openOrPanic := func(filename string) *os.File {
 		y, err := os.OpenFile(filename, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0644)
 		if err != nil {
@@ -17,7 +17,7 @@ func TestNuevo(t *testing.T) {
 		}
 		return y
 	}
-	f := openOrPanic("test/neu.go")
+	f := openOrPanic("_test/neu.go")
 	emitFileHeader(f)
 	emitType := func(tg typeGenerator, w io.Writer) {
 		tg.EmitNodeType(w)