Add modify_cast into geode::cast

[nytelytee]

trafficstars

Allows you to repeatedly apply static_cast in the order the casts are applied in.

So, geode::cast::chain_cast<X, Y, Z>(obj) is equivalent to static_cast<Z>(static_cast<Y>(static_cast<X>(obj))).

Additionally, rarely useful, but geode::cast::chain_cast<>(obj) just returns obj.