Snapshot improvements

[ry]

First pass at simplifying the snapshot interfaces

• Moves how snapshots are supplied to the Isolate. Previously they were given by Behavior::startup_data() but it was only called once at startup. It makes more sense (and simplifies Behavior) to pass it to the constructor of Isolate.
• Adds new libdeno type deno_snapshot instead of overloading deno_buf.
• Adds new libdeno method to delete snapshot deno_snapshot_delete
• Renames deno_get_snapshot to deno_snapshot_new
• Makes StartupData hold references to snapshots. This was implicit when it previously held a deno_buf but is made explicit now. Note that include_bytes!() returns a &'static [u8] and we want to avoid copying that.

[piscisaureus]

The Snapshot1/Snapshot2 thing is not so pretty but I can see that maybe this is necessary.
That said, I'm not sure I have enough detail to really understand where the lifetime issues strike.

Personally I'd try to keep the snapshot ownership in Rust, and wrap it for something like this:

enum SnapShot {
  None,
  Static<&'static [u8]>,
  Owned<Vec<u8>>
}

But I can't tell if that even works when you make the changes you have in mind.
I'd suggest you land it as-is, and i'll look at the complete implementation and tweak it if necessary.

[ry]

@piscisaureus I agree. I really don't want to keep Snapshot1 / Snapshot2 names. I've added a comment that says that. This entire thing is very much incomplete, but since this PR is changing Behavior, I want to go ahead and land it despite the incompleteness.