feat(tvix/eval): implement compilation of upvalue access

This adds a new upvalue tracking structure in the compiler to resolve
upvalues and track their positions within a function when compiling a
closure.

The compiler will emit runtime upvalue access instructions after this
commit, but the creation of the runtime closure object etc. is not yet
wired up.

Change-Id: Ib0c2c25f686bfd45f797c528753068858e3a770d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6289
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/opcode.rs

@@ -13,9 +13,14 @@ pub struct CodeIdx(pub usize);
 
 /// Index of a value in the runtime stack.
 #[repr(transparent)]
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub struct StackIdx(pub usize);
 
+/// Index of an upvalue within a closure's upvalue list.
+#[repr(transparent)]
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct UpvalueIdx(pub usize);
+
 /// Offset by which an instruction pointer should change in a jump.
 #[repr(transparent)]
 #[derive(Clone, Copy, Debug)]
@@ -99,4 +104,5 @@ pub enum OpCode {
 
     // Lambdas
     OpCall,
+    OpGetUpvalue(UpvalueIdx),
 }