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.

[carlbordum]

I added customtype, because the syntax for type structures - while easy and readable - doesn't allow you to express very simple things.

How would I verify that a list only contains 1 integer after this patch?

[carlbordum]

Okay, I see what you're doing with callables. Ill think about it, but may not be available before next weekend.