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>

types/result/result.go

@@ -0,0 +1,49 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package result contains the Of result type, which is
+// either a value or an error.
+package result
+
+// Of is either a T value or an error.
+//
+// Think of it like Rust or Swift's result types.
+// It's named "Of" because the fully qualified name
+// for callers reads result.Of[T].
+type Of[T any] struct {
+	v   T // valid if Err is nil; invalid if Err is non-nil
+	err error
+}
+
+// Value returns a new result with value v,
+// without an error.
+func Value[T any](v T) Of[T] {
+	return Of[T]{v: v}
+}
+
+// Error returns a new result with error err.
+// If err is nil, the returned result is equivalent
+// to calling Value with T's zero value.
+func Error[T any](err error) Of[T] {
+	return Of[T]{err: err}
+}
+
+// MustValue returns r's result value.
+// It panics if r.Err returns non-nil.
+func (r Of[T]) MustValue() T {
+	if r.err != nil {
+		panic(r.err)
+	}
+	return r.v
+}
+
+// Value returns r's result value and error.
+func (r Of[T]) Value() (T, error) {
+	return r.v, r.err
+}
+
+// Err returns r's error, if any.
+// When r.Err returns nil, it's safe to call r.MustValue without it panicking.
+func (r Of[T]) Err() error {
+	return r.err
+}