* 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.
