Refactor symbol parsing for archives so that symbols within the archive itself can be parsed

[moosichu]

This is a tricky one, because ZLD currently requires a file handle + offset. One option is to use this API and then just give it the archive file with the appropriate offset each time. (could work?).

Alternatively we could generate symbols when finalizing and refactor the zld stuff to work in-memory directly (as we have to load the symbol files into memory anyway).

The latter is probably more efficient even if more work, so will need to think on this one.