func (b *Reader) ReadString(delim byte) (string, error) {
	full, frag, n, err := b.collectFragments(delim)
	// Allocate new buffer to hold the full pieces and the fragment.
	var buf strings.Builder
	buf.Grow(n)
	// Copy full pieces and fragment in.
	for _, fb := range full {
		buf.Write(fb)
	}
	buf.Write(frag)
	return buf.String(), err
}

// WriteTo implements io.WriterTo.