Handling of symlinks

[peterwaller-arm]

I think we should find a way to capture symlinks properly. The indices would need a way to represent this - hopefully that would be easy to do with an enum without blowing up the filesize.

[peterwaller-arm]

I would like to address this for 1.0 if possible. It may require further tweaks to the on-disk format so we should possibly do it sooner rather than later. From my recollection there were also issues with the way that we put all paths through canonicalization which meant that we didn't directly observe symlinks. This will need a bit of investigation.