The PR replaces all `goldmark/util.BytesToReadOnlyString` with
`util.UnsafeBytesToString`, `goldmark/util.StringToReadOnlyBytes` with
`util.UnsafeStringToBytes`. This removes one `TODO`.
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
(cherry picked from commit 1761459ebc7eb6d432eced093b4583425a5c5d4b)