Support for Corinna keywords

[davorg]

I raised an issue on Perl::Metrics::Simple because it doesn't recognise that method defines a subroutine. And it turns out that the problem is PPI not supporting method (and, also, class and field).

How hard would it be to support these new keywords? I know nothing about the internals of PPI, but I'd be willing to learn if it would help get this added.