rfctr: upgrade lazyproperty and remove __slots__

Implement @lazyproperty as a descriptor rather than a "hidden" instance
variable like `._propname`. This invalidates a lot of the __slots__
defined to accommodate those hidden instance variables and since
__slots__ was really a mis-step, just get rid of all of them.

Fix instances that took advantage of the presence of that hidden
`._propname` instance variable, particularly very old tests.