`sable snapshot` subcommand

[lino-levan]

We should support AoT compiled JS.

$ bueno snapshot mod.js
$ bueno run mod.snapshot

[lino-levan]

Snapshots will most likely not be stable across bueno versions, so we should definitely note the current bueno version and warn that the snapshot was generated in an old version.

[Im-Beast]

We'd need to think whether we would even support running them not-locally. We'd also need to check if the snapshot has been compiled for current platform.

[Im-Beast]

Also because snapshots would probably be susceptible to memory attacks we should require --allow-all as well as to warn the user about that