feat(tvix/eval): Support builtins.head

TL;DR:
- support `builtins.head`
- define `ErrorKind::IndexOutOfBounds` and canonical error code
- support basic unit tests

Change-Id: I859107ffb4e220cba1be8c2ac41d1913dcca37ff
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6544
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -75,6 +75,14 @@ fn pure_builtins() -> Vec<Builtin> {
             }
             Ok(Value::Integer(args[0].to_list()?.len() as i64))
         }),
+        Builtin::new("head", 1, |args, vm| {
+            force!(vm, &args[0], xs, {
+                match xs.to_list()?.get(0) {
+                    Some(x) => Ok(x.clone()),
+                    None => Err(ErrorKind::IndexOutOfBounds { index: 0 }),
+                }
+            })
+        }),
         Builtin::new("isAttrs", 1, |args, _| {
             Ok(Value::Bool(matches!(args[0], Value::Attrs(_))))
         }),