snapshot `.bin` files should contain metadata

[matthew-levan]

In order to prevent the need to load entire snapshots just to extract metadata (like eve_d, for example), the files should just start with a magic number identifying itself as an Urbit snapshot followed by some metadata. The epoch system might be a good place to add this to our files.

Related #346.

[belisarius222]

I'm thinking it should include a Vere version number, specifically.

[joemfb]

There is a metadata, and it has its own version number. We chose to load the snapshot in the standard way (thereby applying any outstanding patch files), rather than add a code path for reading granularly. But that could be revisited...