Get rid of the need for the .worthy-attributes hack

[masak]

This is an internal thing, but kind of a technical debt issue.

method worthy-attributes {
    sub aname($attr) { $attr.name.substr(2) }
    sub avalue($attr) { $attr.get_value(self) }
    sub worthy($attr) {
        avalue($attr) !~~ Hash  # avoids showing static-lexpad
            && (aname($attr) ne "type" || avalue($attr) ne "")
    }

    return self.^attributes.grep(&worthy);
}

We filter away two things:

• static-lexpad a thing in Q::Statements blocks which we're not even sure belongs there or anywhere, really
• Empty string type (because in the cases when they're empty, they don't add anything)

.worthy-attributes used to be just an inner convenience sub, at which point it was OK and nobody's business. But as part of 690f60a, it got promoted to a method, and this makes it ugly. To me it suggests that both static-lexpad and type should die a gruesome death so the method can be removed.

Removing static-lexpad is the big mystery. It's all tangled up in exactly how much "context" a Qtree really has (or should have). Related tickets are #20 and #47.

Removing type should be dead easy. I think it was there to make some nicer stringification, basically. But after #45 I bet it isn't even used. I could be wrong, though — maybe it's used in custom operator lookup? If it is, then the correct way forward is to allow it on all of them, again making this exception go away.