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/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();