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.

Cargo.toml

@@ -7,3 +7,6 @@ members = [
     "rust/integration-tests",
     "rust/xtask",
 ]
+
+# src/tc/rust is just part of the TW build and not a public crate
+exclude = [ "src/tc/rust" ]