feat(cpp): use cxx (was cbindgen) and mv interface in a workspace

concrete-optimizer-cpp/tests/src/main.cpp

@@ -0,0 +1,11 @@
+#include "concrete-optimizer.hpp"
+
+int main(int argc, char *argv[]) {
+  auto solution = concrete_optimizer::optimise_bootstrap(1, 128, 1, .05);
+
+  if (solution.glwe_polynomial_size != 1024) {
+    return 1;
+  }
+
+  return 0;
+}