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/default.nix

@@ -13,6 +13,10 @@ lib.fix (self: depot.third_party.naersk.buildPackage (lib.fix (naerskArgs: {
 
   meta.ci.targets = builtins.attrNames self.passthru;
 
+  copySources = [
+    "builtin-macros"
+  ];
+
   passthru.benchmarks = depot.third_party.naersk.buildPackage (naerskArgs // {
     name = "tvix-eval-benchmarks";