result::Error should call Printable::disp if the err field implements Printable

[vaivaswatha]

Following up on #34 and #33, it would be nice if, when displaying the err field, if that object can be cast to Any, and that Any object into dyn Printable (via the trait_cast utility), then use that to print the field instead of just Display provided by thiserror.

Today, we try to downcast the err field to result::Error, and in that case, we can call Printable, but not for any other Rust type that implements Printable.

Note: result::Error::err has a as_any() method, but that's the Any object from the outer Box. What we want is (*self.err).as_any() to be used in ``impl Printable for Errorwhich doesn't work today becuasestd::error::Error` provides `downcast` but not `as_any`.