Added convenience methods to type.py.
Passing opt=True to TypedField no longer implies a default value. Updated changelog.
TypedField.copy() should pass in default= as well.
Made TypedField.kind a property.

This ensures that it gets normalized when it's overwritten outside of the constructor.
Clearer error message.
Bugfix nodups kwarg.
Bugfix checktype()/checktype_seq().