refactor(tvix/eval): simplify thunk representations

For now, do not distinguish between closing and non-closing thunks, it
will make the initial implementation easier. See Knuth etc.

Change-Id: I0bd51e0f89f2c77e90bac63b507e5027b649e3d8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6346
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/mod.rs

@@ -877,7 +877,7 @@ impl Compiler {
         // Emit the thunk directly if it does not close over the
         // environment.
         if thunk.lambda.upvalue_count == 0 {
-            self.emit_constant(Value::Thunk(Thunk::new(thunk.lambda)));
+            self.emit_constant(Value::Thunk(Thunk::new(Rc::new(thunk.lambda))));
             return;
         }