feat(tvix/eval): Support builtins.{add,sub}

First pass at supporting `builtins` for tvix. The following tests appear to be
WAI:

```shell
$ cd tvix/eval
$ cargo build
$ cargo test
```

Change-Id: I27cce23d503b17a886d1109e285e8b4be4264977
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6405
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -13,8 +13,15 @@ use crate::{
     value::{Builtin, NixAttrs, NixList, NixString, Value},
 };
 
+use crate::arithmetic_op;
+
 fn pure_builtins() -> Vec<Builtin> {
     vec![
+        Builtin::new("add", 2, |mut args| {
+            let b = args.pop().unwrap();
+            let a = args.pop().unwrap();
+            Ok(arithmetic_op!(a, b, +))
+        }),
         Builtin::new("abort", 1, |mut args| {
             return Err(
                 ErrorKind::Abort(args.pop().unwrap().to_string()?.as_str().to_owned()).into(),
@@ -63,6 +70,11 @@ fn pure_builtins() -> Vec<Builtin> {
         Builtin::new("isString", 1, |args| {
             Ok(Value::Bool(matches!(args[0], Value::String(_))))
         }),
+        Builtin::new("sub", 2, |mut args| {
+            let b = args.pop().unwrap();
+            let a = args.pop().unwrap();
+            Ok(arithmetic_op!(a, b, -))
+        }),
         Builtin::new("throw", 1, |mut args| {
             return Err(
                 ErrorKind::Throw(args.pop().unwrap().to_string()?.as_str().to_owned()).into(),

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-add.exp

@@ -0,0 +1 @@
+[ 18 18.9 18.9 19.1 19 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-add.nix

@@ -0,0 +1,7 @@
+[
+  (builtins.add 7 11)
+  (builtins.add 7.9 11)
+  (builtins.add 7 11.9)
+  (builtins.add 7.2 11.9)
+  (builtins.add 7.1 11.9)
+]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sub.exp

@@ -0,0 +1 @@
+[ -4 -3.1 -4.9 -4.7 -4 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-sub.nix

@@ -0,0 +1,7 @@
+[
+  (builtins.sub 7 11)
+  (builtins.sub 7.9 11)
+  (builtins.sub 7 11.9)
+  (builtins.sub 7.2 11.9)
+  (builtins.sub 7.9 11.9)
+]

tvix/eval/src/vm.rs

@@ -37,6 +37,7 @@ pub struct VM {
     with_stack: Vec<usize>,
 }
 
+#[macro_export]
 macro_rules! arithmetic_op {
     ( $self:ident, $op:tt ) => {{
         let b = $self.pop();