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/columns/CMakeLists.txt

@@ -1,9 +1,11 @@
 cmake_minimum_required (VERSION 3.0)
 include_directories (${CMAKE_SOURCE_DIR}
                      ${CMAKE_SOURCE_DIR}/src
+                     ${CMAKE_SOURCE_DIR}/src/tc
                      ${CMAKE_SOURCE_DIR}/src/commands
                      ${CMAKE_SOURCE_DIR}/src/columns
                      ${CMAKE_SOURCE_DIR}/src/libshared/src
+                     ${CMAKE_SOURCE_DIR}/rust/lib
                      ${TASK_INCLUDE_DIRS})
 
 set (columns_SRCS Column.cpp Column.h