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.

.gitmodules

@@ -1,3 +1,9 @@
 [submodule "src/libshared"]
 	path = src/libshared
 	url = https://github.com/GothenburgBitFactory/libshared.git
+[submodule "cmake/CMakeRust"]
+	path = cmake/CMakeRust
+	url = https://github.com/Devolutions/CMakeRust
+[submodule "src/taskchampion"]
+	path = src/taskchampion
+	url = https://github.com/taskchampion/taskchampion