lang: interfaces, funcs: Add a new graph engine called dage

This adds a new implementation of the function engine that runs the DAG
function graph. This version is notable in that it can run a graph that
changes shape over time. To make changes to the same of the graph, you
must use the new transaction (Txn) system. This system implements a
simple garbage collector (GC) for scheduled removal of nodes that the
transaction system "reverses" out of the graph.

Special thanks to Samuel Gélineau <gelisam@gmail.com> for his help
hacking on and debugging so much of this concurrency work with me.

lang/funcs/dage/util_test.go

@@ -0,0 +1,30 @@
+// Mgmt
+// Copyright (C) 2013-2023+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+//go:build !root
+
+package dage
+
+import (
+	"github.com/purpleidea/mgmt/lang/interfaces"
+)
+
+func testEdge(name string) *interfaces.FuncEdge {
+	return &interfaces.FuncEdge{
+		Args: []string{name},
+	}
+}