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/errors.rs

@@ -25,6 +25,11 @@ pub enum ErrorKind {
         name: String,
     },
 
+    // Attempted to index into a list beyond its boundaries.
+    IndexOutOfBounds {
+        index: usize,
+    },
+
     TypeError {
         expected: &'static str,
         actual: &'static str,
@@ -123,6 +128,10 @@ impl Error {
                 name
             ),
 
+            ErrorKind::IndexOutOfBounds { index } => {
+                format!("list index '{}' is out of bounds", index)
+            }
+
             ErrorKind::TypeError { expected, actual } => format!(
                 "expected value of type '{}', but found a '{}'",
                 expected, actual
@@ -208,6 +217,7 @@ to a missing value in the attribute set(s) included via `with`."#,
             ErrorKind::DuplicateAttrsKey { .. } => "E016",
             ErrorKind::ThunkForce(_) => "E017",
             ErrorKind::NotCoercibleToString { .. } => "E018",
+            ErrorKind::IndexOutOfBounds { .. } => "E019",
             ErrorKind::NotImplemented(_) => "E999",
         }
     }