libexpr: add expr memory management

with the prepatory work done this mostly means turning plain pointers
into unique_ptrs, with all the associated churn that necessitates. we
might want to change some of these to box_ptrs at some point as well,
but that would be a semantic change that isn't fully appropriate yet.

Change-Id: I0c238c118617420650432f4ed45569baa3e3f413

src/nix/flake.cc

@@ -446,7 +446,7 @@ struct CmdFlakeCheck : FlakeCommand
                 if (v.lambda.fun->hasFormals()
                     || !argHasName(v.lambda.fun->arg, "final"))
                     throw Error("overlay does not take an argument named 'final'");
-                auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body);
+                auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body.get());
                 if (!body
                     || body->hasFormals()
                     || !argHasName(body->arg, "prev"))