Add a C++ wrapper around TC FFI

This uses CMake to build a simple Rust library (in `src/tc/rust`) that
just re-exports everything from the `taskchampion-lib` crate.

The C++ wrappers then wrap this into C++ objects with proper lifecycle
maintenance, in the `tc` namespace.

The C++ wrappers are incomplete, and missing methods are tagged with
"TODO".  These will be added as needed.

src/tc/CMakeLists.txt

@@ -0,0 +1,16 @@
+cmake_minimum_required (VERSION 3.0)
+include_directories (${CMAKE_SOURCE_DIR}
+                     ${CMAKE_SOURCE_DIR}/src
+                     ${CMAKE_SOURCE_DIR}/src/tc
+                     ${CMAKE_SOURCE_DIR}/src/libshared/src
+                     ${CMAKE_SOURCE_DIR}/rust/lib
+                     ${TASK_INCLUDE_DIRS})
+
+set (tc_SRCS
+  ffi.h
+  util.cpp util.h
+  Replica.cpp Replica.h
+  WorkingSet.cpp WorkingSet.h
+  Task.cpp Task.h)
+
+add_library (tc STATIC ${tc_SRCS})