refactor(tvix/eval): encapsulate internal mutability within Closure

This is required to efficiently construct the upvalue array at
runtime, as there are situations where during Closure construction
multiple things already have a reference to the closure (e.g. a
self-reference).

Change-Id: I35263b845fdc695dc873de489f5168d39b370f6a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6312
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/mod.rs

@@ -87,8 +87,7 @@ impl Compiler {
     }
 
     fn chunk(&mut self) -> &mut Chunk {
-        Rc::<Chunk>::get_mut(self.context_mut().lambda.chunk())
-            .expect("compiler flaw: long-lived chunk reference")
+        &mut self.context_mut().lambda.chunk
     }
 
     fn scope(&self) -> &Scope {