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/chunk.rs

@@ -1,5 +1,5 @@
 use std::io::Write;
-use std::ops::Index;
+use std::ops::{Index, IndexMut};
 
 use crate::opcode::{CodeIdx, ConstantIdx, OpCode};
 use crate::value::Value;
@@ -51,6 +51,12 @@ impl Index<CodeIdx> for Chunk {
     }
 }
 
+impl IndexMut<CodeIdx> for Chunk {
+    fn index_mut(&mut self, index: CodeIdx) -> &mut Self::Output {
+        &mut self.code[index.0]
+    }
+}
+
 impl Chunk {
     pub fn push_op(&mut self, data: OpCode, span: codemap::Span) -> CodeIdx {
         let idx = self.code.len();