types/result, util/lineiter: add package for a result type, use it

This adds a new generic result type (motivated by golang/go#70084) to
try it out, and uses it in the new lineutil package (replacing the old
lineread package), changing that package to return iterators:
sometimes over []byte (when the input is all in memory), but sometimes
iterators over results of []byte, if errors might happen at runtime.

Updates #12912
Updates golang/go#70084

Change-Id: Iacdc1070e661b5fb163907b1e8b07ac7d51d3f83
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/lineiter/lineiter_test.go

@@ -0,0 +1,32 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package lineiter
+
+import (
+	"slices"
+	"strings"
+	"testing"
+)
+
+func TestBytesLines(t *testing.T) {
+	var got []string
+	for line := range Bytes([]byte("foo\n\nbar\nbaz")) {
+		got = append(got, string(line))
+	}
+	want := []string{"foo", "", "bar", "baz"}
+	if !slices.Equal(got, want) {
+		t.Errorf("got %q; want %q", got, want)
+	}
+}
+
+func TestReader(t *testing.T) {
+	var got []string
+	for line := range Reader(strings.NewReader("foo\n\nbar\nbaz")) {
+		got = append(got, string(line.MustValue()))
+	}
+	want := []string{"foo", "", "bar", "baz"}
+	if !slices.Equal(got, want) {
+		t.Errorf("got %q; want %q", got, want)
+	}
+}