Add builtins.fetchClosure

This allows closures to be imported at evaluation time, without
requiring the user to configure substituters. E.g.

  builtins.fetchClosure {
    storePath = /nix/store/f89g6yi63m1ywfxj96whv5sxsm74w5ka-python3.9-sqlparse-0.4.2;
    from = "https://cache.ngi0.nixos.org";
  }

src/libexpr/primops/fetchTree.cc

@@ -145,7 +145,7 @@ static void fetchTree(
         if (!params.allowNameArgument)
             if (auto nameIter = attrs.find("name"); nameIter != attrs.end())
                 throw Error({
-                    .msg = hintfmt("attribute 'name' isn’t supported in call to 'fetchTree'"),
+                    .msg = hintfmt("attribute 'name' isn't supported in call to 'fetchTree'"),
                     .errPos = pos
                 });
 
@@ -329,7 +329,7 @@ static RegisterPrimOp primop_fetchTarball({
     .fun = prim_fetchTarball,
 });
 
-static void prim_fetchGit(EvalState &state, const Pos &pos, Value **args, Value &v)
+static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     fetchTree(state, pos, args, v, "git", FetchTreeParams { .emptyRevFallback = true, .allowNameArgument = true });
 }