feat(tvix/eval): Support builtins.elemAt

(Attempt to) index into a list.

Change-Id: I3592e60a79e64d265e34100d4062041b0b410e00
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6551
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -26,9 +26,9 @@ pub enum ErrorKind {
         name: String,
     },
 
-    // Attempted to index into a list beyond its boundaries.
+    /// Attempted to index into a list beyond its boundaries.
     IndexOutOfBounds {
-        index: usize,
+        index: i64,
     },
 
     TypeError {