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/Replica.h

@@ -0,0 +1,85 @@
+////////////////////////////////////////////////////////////////////////////////
+//
+// 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_REPLICA
+#define INCLUDED_TC_REPLICA
+
+#include <string>
+#include <functional>
+#include <memory>
+#include <optional>
+#include "tc/ffi.h"
+#include "tc/Task.h"
+
+namespace tc {
+  class Task;
+  class WorkingSet;
+
+  // a unique_ptr to a TCReplica which will automatically free the value when
+  // it goes out of scope.
+  using unique_tcreplica_ptr = std::unique_ptr<
+    tc::ffi::TCReplica,
+    std::function<void(tc::ffi::TCReplica*)>>;
+
+  // Replica wraps the TCReplica type, managing its memory, errors, and so on.
+  //
+  // Except as noted, method names match the suffix to `tc_replica_..`.
+  class Replica
+  {
+  public:
+    Replica (); // tc_replica_new_in_memory
+    Replica (const std::string& dir); // tc_replica_new_on_disk
+
+    // This object "owns" inner, so copy is not allowed.
+    Replica (const Replica &) = delete;
+    Replica &operator=(const Replica &) = delete;
+
+    // Explicit move constructor and assignment
+    Replica (Replica &&) noexcept;
+    Replica &operator=(Replica &&) noexcept;
+
+    std::vector<tc::Task> all_tasks ();
+// TODO: struct TCUuidList tc_replica_all_task_uuids(struct TCReplica *rep);
+    tc::WorkingSet working_set ();
+    std::optional<tc::Task> get_task (const std::string &uuid);
+    tc::Task new_task (Status status, const std::string &description);
+// TODO: struct TCTask *tc_replica_import_task_with_uuid(struct TCReplica *rep, struct TCUuid tcuuid);
+// TODO: TCResult tc_replica_sync(struct TCReplica *rep, struct TCServer *server, bool avoid_snapshots);
+// TODO: TCResult tc_replica_undo(struct TCReplica *rep, int32_t *undone_out);
+// TODO: TCResult tc_replica_add_undo_point(struct TCReplica *rep, bool force);
+    void rebuild_working_set ();
+  private:
+    unique_tcreplica_ptr inner;
+
+    // construct an error message from tc_replica_error, or from the given
+    // string retrieved from tc_replica_error.
+    std::string replica_error ();
+    std::string replica_error (tc::ffi::TCString string);
+  };
+}
+
+#endif
+////////////////////////////////////////////////////////////////////////////////