Add API to create a Snapshot without going through creating a VM partition

[jprendes]

Ok, I think the way to go would be to add a new builder terminator snapshot_from_file and snapshot_from_bytes, and use those to benchmark all the setup that we were measuring for the uninitialized sandbox here (maybe except for the memory allocation and memcpy, but that's not a big loss).

I'll create an issue to add this and reintroduce the new benchmarks to compensate what we loose here.

Originally posted by @jprendes in #1746 (comment)

[syntactically]

I'm not totally convinced this is a good idea, because the existence of uninitialized snapshots is kind of a nasty hack the way it is done now---I don't think there is ever a good use case for them. So, I've been thinking it might make sense to move towards needing a partition handle for snapshot creation so that we can do the initialize call inside of Snapshot::from_file.

[yoshuawuyts]

Snapshot::from_file would also match the SandboxBuilder::from_file API discussed in #1748

[ludfjig]

Add API to create a Snapshot without going through creating a VM partition

I also think we don't want this. It would be really nice if we can guarantee that all snapshots have gone through VM initialization. For benchmarking, then the only thing to track are

1. Time to create an initialized snapshot (from binary) (this includes vm init time)
2. Time to create a sandbox from initialized snapshot (this includes creating vm from scratch for disk-snapshot, and possibly for in-memory snapshtos as well)

[jprendes]

Can do so the initializing when we create the sandbox from an uninitiated snapshot?

[ludfjig]

Can do so the initializing when we create the sandbox from an uninitiated snapshot?

That's what we do right now