WISHLIST: Add signature() method to PPI::Statement::Sub

[trwyant]

When drafting a Perl::Critic policy to forbid @_ and @ARG in subroutines with signatures, it would be helpful (though obviously not essential) to have PPI::Statement::Sub->signature(). I believe that (unlike PPI::Statement::Sub->prototype()) it should return the actual PPI::Statement::Signature object if it exists, rather than just the content of the object. But my use case just needs to know whether the subroutine has a signature, so any return whose Boolean value contains this information works for me.

I am willing to do a pull request for any such functionality that we can agree on.