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.

src/CMakeLists.txt

@@ -25,6 +25,7 @@ set (task_SRCS API.cpp API.h
                Path.cpp Path.h
                Permission.cpp Permission.h
                Record.cpp Record.h
+               RegX.cpp RegX.h
                TDB.cpp TDB.h
                TDB2.cpp TDB2.h
                Task.cpp Task.h
@@ -45,7 +46,6 @@ set (task_SRCS API.cpp API.h
                interactive.cpp
                recur.cpp
                rules.cpp
-               rx.cpp rx.h
                sort.cpp
                text.cpp text.h
                utf8.cpp utf8.h