feat(tvix/eval): Add a proc-macro for defining builtins

Add a single new proc macro to a new proc-macro crate,
`tvix-eval-proc-macros` for defining an inline module containing nix
builtins, and automatically generating a function within that module
which returns a list of those builtins as `tvix_eval::value::Builtin`.

Change-Id: Ie4afae438914d2af93d15637151a49b4c68aa352
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7198
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI

tvix/eval/builtin-macros/tests/tests.rs

@@ -0,0 +1,26 @@
+use tvix_eval_builtin_macros::builtins;
+
+mod value {
+    pub use tvix_eval::Builtin;
+}
+
+#[builtins]
+mod builtins {
+    use tvix_eval::{ErrorKind, Value, VM};
+
+    #[builtin("identity")]
+    pub fn builtin_identity(_vm: &mut VM, x: Value) -> Result<Value, ErrorKind> {
+        Ok(x)
+    }
+
+    #[builtin("tryEval")]
+    pub fn builtin_try_eval(_: &mut VM, #[lazy] _x: Value) -> Result<Value, ErrorKind> {
+        todo!()
+    }
+}
+
+#[test]
+fn builtins() {
+    let builtins = builtins::builtins();
+    assert_eq!(builtins.len(), 2);
+}