feat(tvix/eval): add initial representation of builtins

Builtins are represented as a Rust function pointer that accepts a
vector of arguments, which represents variable arity builtins.

Change-Id: Ibab7e662a646caf1172695d876d2f55e187c03dd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6251
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/vm.rs

@@ -358,6 +358,11 @@ impl VM {
                     let callable = self.pop();
                     match callable {
                         Value::Lambda(lambda) => self.call(lambda, 1),
+                        Value::Builtin(builtin) => {
+                            let arg = self.pop();
+                            let result = builtin.apply(arg)?;
+                            self.push(result);
+                        }
                         _ => return Err(ErrorKind::NotCallable.into()),
                     };
                 }