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.

.gitignore

@@ -5,6 +5,7 @@ src/task
 src/taskd
 src/libtask.a
 src/commands/libcommands.a
+src/tc/libtc.a
 src/columns/libcolumns.a
 *~
 .*.swp