Instead of setting `-std` compiler flag directly, set the
`CMAKE_CXX_STANDARD` variable. This lets CMake know the required C++
standard and evaluate the final compiler flag correctly, taking into
account compile features set by `target_compile_features()`.
This change preserves the existing behavior, where compiler extensions
are disabled for other targets than Cygwin.
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.
