class-construction: Some typos fixed, and some nitpicking

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'

Perl-Apollo / Corinna