refactor(tvix/eval): use CodeIdx wrapper for instruction pointer

As suggested by sterni in cl/6453.

Change-Id: I3cf80d97c11fd7d085ab510f6be4b5f937c791ec
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6562
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/opcode.rs

@@ -1,6 +1,8 @@
 //! This module implements the instruction set running on the abstract
 //! machine implemented by tvix.
 
+use std::ops::{AddAssign, Sub};
+
 /// Index of a constant in the current code chunk.
 #[repr(transparent)]
 #[derive(Clone, Copy, Debug)]
@@ -11,6 +13,20 @@ pub struct ConstantIdx(pub usize);
 #[derive(Clone, Copy, Debug)]
 pub struct CodeIdx(pub usize);
 
+impl AddAssign<usize> for CodeIdx {
+    fn add_assign(&mut self, rhs: usize) {
+        *self = CodeIdx(self.0 + rhs)
+    }
+}
+
+impl Sub<usize> for CodeIdx {
+    type Output = Self;
+
+    fn sub(self, rhs: usize) -> Self::Output {
+        CodeIdx(self.0 - rhs)
+    }
+}
+
 /// Index of a value in the runtime stack.
 #[repr(transparent)]
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd)]