Double negation '(unless (! ...))' replaced by 'if'
Interpolated scalars in error messages are enclosed in single quotes
(at this point they could be objects with overloaded stringification
and totally mess up the error message)
In "Find constructor args" allow different classes in the MRO to
do the same role (so their constructor args only count once)
In the MOP stuff (should this be part of the RFC? It isn't complete)
reflect that 'new' has been renamed to ':param'