Task: Remove std::map inheritance, clean up interface

- Make the Task object's interface more explicit by removing the
  std::map inheritance.
- Using this more explicit interface, remove unneeded ctors in order to
  allow the compiler to "Do The Right Thing"(tm).
  This leads to a performance improvement of 12% in the "add"
  performance test, and 7% for "import".

test/t.t.cpp

@@ -189,7 +189,7 @@ TODO Task::decode
   test.is (task.get ("three"), "four", "three=four");
 
   // Task::set
-  task.clear ();
+  task.data.clear ();
   task.set ("name", "value");
   test.is (task.composeF4 (), "[name:\"value\"]", "Task::set");
 
@@ -213,7 +213,7 @@ TODO Task::decode
   test.is (task.composeF4 (), "[name:\"value\"]", "Task::remove");
 
   // Task::all
-  test.is (task.size (), (size_t)1, "Task::all size");
+  test.is (task.data.size (), (size_t)1, "Task::all size");
 
   ////////////////////////////////////////////////////////////////////////////////
 

test/util.t.cpp

@@ -78,7 +78,7 @@ int main (int, char**)
   Task rightAgain (right);
 
   std::string output = taskDifferences (left, right);
-  t.ok (left != right,                                                              "Detected changes");
+  t.ok (left.data != right.data,                                                              "Detected changes");
   t.ok (output.find ("Zero will be changed from '0' to '00'") != std::string::npos, "Detected change zero:0 -> zero:00");
   t.ok (output.find ("One will be deleted")                   != std::string::npos, "Detected deletion one:1 ->");
   t.ok (output.find ("Two")                                   == std::string::npos, "Detected no change two:2 -> two:2");