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/tests/tvix_tests/eval-okay-builtins-head.exp

@@ -0,0 +1 @@
+[ "foo" 1 ]

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

@@ -0,0 +1,4 @@
+[
+  (builtins.head [ "foo" ])
+  (builtins.head [ 1 2 3 ])
+]