starlark: add example of Starlark built-in function in Go (google#217)

Fixes google#216

starlark/example_test.go

@@ -23,17 +23,35 @@ import (
 func ExampleExecFile() {
 	const data = `
 print(greeting + ", world")
-
+print(repeat("one"))
+print(repeat("mur", 2))
 squares = [x*x for x in range(10)]
 `
 
+	// repeat(str, n=1) is a Go function called from Starlark.
+	// It behaves like the 'string * int' operation.
+	repeat := func(thread *starlark.Thread, b *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
+		var s string
+		var n int = 1
+		if err := starlark.UnpackArgs(b.Name(), args, kwargs, "s", &s, "n?", &n); err != nil {
+			return nil, err
+		}
+		return starlark.String(strings.Repeat(s, n)), nil
+	}
+
+	// The Thread defines the behavior of the built-in 'print' function.
 	thread := &starlark.Thread{
 		Name:  "example",
 		Print: func(_ *starlark.Thread, msg string) { fmt.Println(msg) },
 	}
+
+	// This dictionary defines the pre-declared environment.
 	predeclared := starlark.StringDict{
 		"greeting": starlark.String("hello"),
+		"repeat":   starlark.NewBuiltin("repeat", repeat),
 	}
+
+	// Execute a program.
 	globals, err := starlark.ExecFile(thread, "apparent/filename.star", data, predeclared)
 	if err != nil {
 		if evalErr, ok := err.(*starlark.EvalError); ok {
@@ -51,6 +69,8 @@ squares = [x*x for x in range(10)]
 
 	// Output:
 	// hello, world
+	// one
+	// murmur
 	//
 	// Globals:
 	// squares (list) = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]