Make modify_cast more powerful

[nytelytee]

The original modify_cast was only able to cast a non-modify class to a modify class.

This pull request enables it to do the reverse, cast from a modify class to a non-modify class as well.

It also enables it to cast between 2 modify classes.

[nytelytee]

One additional question I forgot:
would you be fine with me also adding a modify_typeinfo_cast that will do a typeinfo_cast at the right place in the cast chain, so that one can also use this at runtime? I think that at each of the 3 branches, only 1 typeinfo_cast is necessary, when switching between base types. The requires from the static asserts in the branches can also be removed in that case, I think.

Also the void checks inside of the static asserts in the branches may be redundant given that the concept in the if constexpr already checks for that. Hence, in the typeinfo_cast version, the entire static asserts could be removed altogether.

[nytelytee]

Does Geode have a style guide for concept names? I named a concept IsRealModify, but am not sure if I should have used snake_case or not.

[dankmeme01]

looking at source it's inconsistent but mostly PascalCase