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.cpp

@@ -0,0 +1,98 @@
+////////////////////////////////////////////////////////////////////////////////
+//
+// 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
+//
+////////////////////////////////////////////////////////////////////////////////
+
+#include <cmake.h>
+#include <format.h>
+#include "tc/WorkingSet.h"
+#include "tc/Task.h"
+#include "tc/util.h"
+
+using namespace tc::ffi;
+
+////////////////////////////////////////////////////////////////////////////////
+tc::WorkingSet::WorkingSet (WorkingSet &&other) noexcept
+{
+  // move inner from other
+  inner = unique_tcws_ptr (
+      other.inner.release (),
+      [](TCWorkingSet* ws) { tc_working_set_free (ws); });
+}
+
+////////////////////////////////////////////////////////////////////////////////
+tc::WorkingSet& tc::WorkingSet::operator= (WorkingSet &&other) noexcept
+{
+  if (this != &other) {
+    // move inner from other
+    inner = unique_tcws_ptr (
+        other.inner.release (),
+        [](TCWorkingSet* ws) { tc_working_set_free (ws); });
+  }
+  return *this;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+tc::WorkingSet::WorkingSet (tc::ffi::TCWorkingSet* tcws)
+{
+  inner = unique_tcws_ptr (
+      tcws,
+      [](TCWorkingSet* ws) { tc_working_set_free (ws); });
+}
+
+////////////////////////////////////////////////////////////////////////////////
+size_t tc::WorkingSet::len () const noexcept
+{
+  return tc_working_set_len (&*inner);
+}
+
+////////////////////////////////////////////////////////////////////////////////
+size_t tc::WorkingSet::largest_index () const noexcept
+{
+  return tc_working_set_largest_index (&*inner);
+}
+
+////////////////////////////////////////////////////////////////////////////////
+std::optional<std::string> tc::WorkingSet::by_index (size_t index) const noexcept
+{
+  TCUuid uuid;
+  if (tc_working_set_by_index (&*inner, index, &uuid)) {
+    return std::make_optional (tc2uuid (uuid));
+  } else {
+    return std::nullopt;
+  }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+std::optional<size_t> tc::WorkingSet::by_uuid (const std::string &uuid) const noexcept
+{
+  auto index = tc_working_set_by_uuid (&*inner, uuid2tc (uuid));
+  if (index > 0) {
+    return std::make_optional (index);
+  } else {
+    return std::nullopt;
+  }
+}
+
+////////////////////////////////////////////////////////////////////////////////