Added support for entry insertion

Currently unused, the insertion of new file entries in arbitrary
locations in a metadata-pair is very easy to add into the existing
metadata logging.

The only tricky things:
1. Name tags must strictly precede any tags related to a file. We can
   pull this off during a compact, but must make two passes. One for the
   name tag, one for the file. Though a benefit of this is that now our
   scans during moves can exit early upon finding the name tag.

1. We need to handle name tags appearing out of order. This makes name
   tags symmetric to deletes, although it doesn't seem like we can
   leverage this fact very well. Note this also means we need to make
   the superblock tag a type of name tag.