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/util.h

@@ -0,0 +1,51 @@
+////////////////////////////////////////////////////////////////////////////////
+//
+// Copyright 2022, Dustin J. Mitchell
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+// THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+//
+// https://www.opensource.org/licenses/mit-license.php
+//
+////////////////////////////////////////////////////////////////////////////////
+
+#ifndef INCLUDED_TC_UTIL
+#define INCLUDED_TC_UTIL
+
+#include <string>
+#include "tc/ffi.h"
+
+namespace tc {
+  // convert a std::string into a TCString, copying the contained data
+  tc::ffi::TCString string2tc(const std::string&);
+
+  // convert a TCString into a std::string, leaving the TCString as-is
+  std::string tc2string_clone(const tc::ffi::TCString&);
+
+  // convert a TCString into a std::string, freeing the TCString
+  std::string tc2string(tc::ffi::TCString&);
+
+  // convert a TCUuid into a std::string
+  std::string tc2uuid(tc::ffi::TCUuid&);
+
+  // parse a std::string into a TCUuid (throwing if parse fails)
+  tc::ffi::TCUuid uuid2tc(const std::string&);
+}
+
+#endif
+////////////////////////////////////////////////////////////////////////////////