feat(tvix/eval): allow extending builtins outside of tvix_eval

The change allows applications that use tvix_serde for parsing
nix-based configuration to extend the language with domain-specific
set of features.

Change-Id: Ia86612308a167c456ecf03e93fe0fbae55b876a6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8848
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/tests/mod.rs

@@ -10,6 +10,7 @@ mod one_offs;
 mod mock_builtins {
     //! Builtins which are required by language tests, but should not
     //! actually exist in //tvix/eval.
+    use crate as tvix_eval;
     use crate::generators::GenCo;
     use crate::*;
     use genawaiter::rc::Gen;