Nice way to allow user to specify attrs on derived items

Often a d-d macro generates a derived type.

The d-d macro's user is likely to want the derived type to implement various traits (Debug maybe). Sometimes this can be done unconditionally by the macro, in which case the macro should do so.

But sometimes it ought to be conditional, or the macro doesn't know about the trait(s).

There should be a more convenient way to do this than

#[derive(Deftly)]
#[derive_deftly(Thingy)]
#[deftly(thingy(attrs = "#[derive(Debug)]"))]
struct Foo { }

and

${if tmeta(thingy(attrs)) { ${tmeta(thingy(attrs)) as token_stream} }}
struct $<$tname Derived> {..

CC @opara