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/vm.rs

@@ -366,6 +366,8 @@ impl VM {
                         _ => return Err(ErrorKind::NotCallable.into()),
                     };
                 }
+
+                OpCode::OpGetUpvalue(_) => todo!("getting upvalues"),
             }
 
             #[cfg(feature = "disassembler")]