Improve handling of timestamps and headers, with explicit errors for old rip format

[MilesCranmer]

This also:

1. Refactors a lot of the Record and RecordItem structs to eliminate code reuse.
2. Removes the extra file creation step within write_log to be similar in function to the other steps.

This fixes #67 by giving a more helpful error message for inability to parse the date format due to usage of old rip.

@chrisgeo do you think this would be worth it? It would also be helpful if you could test it on your "real-world" corrupted .record! I try to simulate a couple but want to make sure it works for yours as well.

[chrisgeo]

Your test case looks to cover it all. I’ll see if I can get my rust env setup to compile.