Regex

- Implemented RegX class to maintain a separate compile, and match
  method, thereby allowing efficient re-use of the regex.  This is
  critical to Expression::eval, where an identical regex might be
  applied to every task.
- Obsoleted rx.{h,cpp}, which combined the compile and match steps
  into a single call, and is therefore not efficient when used in
  the context of filtering.
- Fixed some unit tests that weren't building.  Now they do.  They
  don't work of course (don't be silly) but that's a problem for
  another day.
- Modified all code that relies on rx.h to use RegX.h.

test/tdb2.t.cpp

@@ -38,9 +38,8 @@ void get (std::vector <Task>& pending, std::vector <Task>& completed)
   TDB tdb;
   tdb.location (".");
   tdb.lock ();
-  Filter filter;
-  tdb.loadPending   (pending,   filter);
-  tdb.loadCompleted (completed, filter);
+  tdb.loadPending   (pending);
+  tdb.loadCompleted (completed);
   tdb.unlock ();
 }