Headers: use 1 array rather than 2

Currently, Headers uses 1 array for ids and 1 for the actual headers. Experiment whether joining them into one array makes sense, e.g.:

| id-1 | hdr-1 | id-2 | hdr-2 | ... | id-n | hdr-n |

This saves 4 bytes (compressed OOPs) for an array ref, plus the overhead of the array itself. Plus, resizing only has to be applied to 1 array.