feat(tvix/eval): deduplicate overlap between Closure and Thunk

This commit deduplicates the Thunk-like functionality from Closure
and unifies it with Thunk.

Specifically, we now have one and only one way of breaking reference
cycles in the Value-graph: Thunk.  No other variant contains a
RefCell.  This should make it easier to reason about the behavior of
the VM.  InnerClosure and UpvaluesCarrier are no longer necessary.

This refactoring allowed an improvement in code generation:
`Rc<RefCell<>>`s are now created only for closures which do not have
self-references or deferred upvalues, instead of for all closures.
OpClosure has been split into two separate opcodes:

- OpClosure creates non-recursive closures with no deferred
  upvalues.  The VM will not create an `Rc<RefCell<>>` when executing
  this instruction.

- OpThunkClosure is used for closures with self-references or
  deferred upvalues.  The VM will create a Thunk when executing this
  opcode, but the Thunk will start out already in the
  `ThunkRepr::Evaluated` state, rather than in the
  `ThunkRepr::Suspeneded` state.

To avoid confusion, OpThunk has been renamed OpThunkSuspended.

Thanks to @sterni for suggesting that all this could be done without
adding an additional variant to ThunkRepr.  This does however mean
that there will be mutating accesses to `ThunkRepr::Evaluated`,
which was not previously the case.  The field `is_finalised:bool`
has been added to `Closure` to ensure that these mutating accesses
are performed only on finalised Closures.  Both the check and the
field are present only if `#[cfg(debug_assertions)]`.

Change-Id: I04131501029772f30e28da8281d864427685097f
Signed-off-by: Adam Joseph <adam@westernsemico.com>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7019
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/compiler/scope.rs

@@ -50,6 +50,9 @@ pub struct Local {
     /// Does this local need to be finalised after the enclosing scope
     /// is completely constructed?
     pub needs_finaliser: bool,
+
+    /// Does this local's upvalues contain a reference to itself?
+    pub must_thunk: bool,
 }
 
 impl Local {
@@ -228,6 +231,7 @@ impl Scope {
             name: LocalName::Phantom,
             depth: self.scope_depth,
             needs_finaliser: false,
+            must_thunk: false,
             used: true,
         });
 
@@ -243,6 +247,7 @@ impl Scope {
             depth: self.scope_depth,
             initialised: false,
             needs_finaliser: false,
+            must_thunk: false,
             used: false,
         });
 
@@ -259,6 +264,12 @@ impl Scope {
         self.locals[idx.0].needs_finaliser = true;
     }
 
+    /// Mark local as must be wrapped in a thunk.  This happens if
+    /// the local has a reference to itself in its upvalues.
+    pub fn mark_must_thunk(&mut self, idx: LocalIdx) {
+        self.locals[idx.0].must_thunk = true;
+    }
+
     /// Compute the runtime stack index for a given local by
     /// accounting for uninitialised variables at scopes below this
     /// one.