Deprecate the `@customtype` decorator

[illright]

I don't think it's necessary to set attributes for type-checking functions or decorate them in any way.

I've kept the @customtype decorator for backwards compatibility, but the __datatyping_validate attribute is no longer checked. Now any callable is accepted as a type checker.

I've also refactored the code a bit.

Let me know what you think about this change.