Support using = self.inner for Debug

[dhardy]

There are instances in the std library where a Debug implementation forwards to a single field, e.g. the impl for File. We don't currently support forwarding for Debug, but I think we could support this, e.g.:

# use impl_tools::autoimpl;
#[autoimpl(Debug using self.0)]
struct Thing<T: std::fmt::Debug>(T);

[dhardy]

This impl would print the wrong type name.