wip: stack-graph integration

[oppiliappan]

this implementation currently un-does a bunch of good work:

• disables scope-graph based navigation
• disables bincode serialization and uses json serialization instead
• disables the /answer endpoint altogether

the memoization also makes the assumption that the underlying implementation of StackGraphLanguage is safe to Send (even when marked as !Send), see comment for more. this seems perfectly okay for now, but it would be good to verify this.