Generally just define things in the order they are declared (will make a
change to use ByteString in this file a bit easier to follow). Also make
a couple of free functions be class methods on Header / HeaderList.
Disallow calling `StringBase::bytes()` on temporaries to avoid returning
`ReadonlyBytes` that outlive the underlying string.
With this change, we catch a real UAF:
`load_result.data = maybe_response.release_value().bytes();`
All other updated call sites were already safe, they just needed to use
an intermediate named variable to satisfy the new lvalue-only
requirement.
