Add serialization/deserialization hook support for structs/unions

[RossComputerGuy]

Similarly to how std.fmt supports adding a format() function to a struct or union, s2s should support it for custom serialization/deserialization or for when a type isn't fully supported. This would help with unsupported types that can't easily be serialized due to recursiveness.

[ikskuh]

I want to change s2s format in a breaking way so that arbitrary graphs can be serialized instead of a pure tree structure, so this issue will be resolved. Then, all runtime viable types are actually serializable with s2s except for a [*]T