Directly call source.provide instead of going through dyn error

[dtolnay]

From https://github.com/dtolnay/thiserror/blob/1.0.33/src/aserror.rs, the .as_dyn_error() method exists if the #[source] has a std::error::Error impl, or derefs to something with a std::error::Error impl, or derefs to dyn Error + … (such as Box<dyn Error> would do).

None of those cases require the thiserror-generated provide method to actually go through dyn Error. We can directly call the provide method, which will do the deref'ing and then call provide from either the type's std::error::Error impl or from dyn Error's vtable.

This wasn't possible prior to #182 because .backtrace() couldn't have been called directly the same way, since anyhow::Error has a backtrace method with a different signature than std::error::Error's backtrace method (-> &Backtrace vs -> Option<&Backtrace>).