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/WorkingSet.h

@@ -0,0 +1,75 @@
+////////////////////////////////////////////////////////////////////////////////
+//
+// 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_WORKINGSET
+#define INCLUDED_TC_WORKINGSET
+
+#include <string>
+#include <functional>
+#include <memory>
+#include <optional>
+#include "tc/ffi.h"
+#include "tc/Task.h"
+
+namespace tc {
+  class Task;
+
+  // a unique_ptr to a TCWorkingSet which will automatically free the value when
+  // it goes out of scope.
+  using unique_tcws_ptr = std::unique_ptr<
+    tc::ffi::TCWorkingSet,
+    std::function<void(tc::ffi::TCWorkingSet*)>>;
+
+  // WorkingSet wraps the TCWorkingSet type, managing its memory, errors, and so on.
+  //
+  // Except as noted, method names match the suffix to `tc_working_set_..`.
+  class WorkingSet
+  {
+  protected:
+    friend class tc::Replica;
+    WorkingSet (tc::ffi::TCWorkingSet*); // via tc_replica_working_set
+
+  public:
+    // This object "owns" inner, so copy is not allowed.
+    WorkingSet (const WorkingSet &) = delete;
+    WorkingSet &operator=(const WorkingSet &) = delete;
+
+    // Explicit move constructor and assignment
+    WorkingSet (WorkingSet &&) noexcept;
+    WorkingSet &operator=(WorkingSet &&) noexcept;
+
+    size_t len () const noexcept; // tc_working_set_len
+    size_t largest_index () const noexcept; // tc_working_set_largest_index
+    std::optional<std::string> by_index (size_t index) const noexcept; // tc_working_set_by_index
+    std::optional<size_t> by_uuid (const std::string &index) const noexcept; // tc_working_set_by_uuid
+
+  private:
+    unique_tcws_ptr inner;
+  };
+}
+
+#endif
+////////////////////////////////////////////////////////////////////////////////