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/rust/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "tc-rust"
+version = "0.1.0"
+
+[lib]
+crate-type = ["staticlib"]
+
+[dependencies]
+taskchampion-lib = {path = "../../../rust/lib"}