SPCurve::smart_pointer

Implement std::default_delete<SPCurve> which preserves the refcount logic when using std::unique_ptr<SPCurve>.

The preferred pattern to manage new or reference counted SPCurve instances should be:

auto curve1 = std::make_unique<SPCurve>();
auto curve2 = curve1->ref();
// both instances will unref automatically when they go out of scope

Direct calls to SPCurve::unref() are deprecated.

Fixes some memory leaks in the node tool.