When I'm Sixty-Four

⚠️ Breaking change
node::integer is now std::int64_t and node::decimal is now std::float64_t (or double where std::float64_t isn't available), on every platform.

🎵 Will you still need me, will you still feed me
Code that names the old types explicitly, e.g. std::get< long> or std::holds_alternative< double>, must be updated.

Internal cleanup with fixed-width type aliases, fewer name collisions, and no more aliases injected into namespace std (which the previous release did).