Add Error::source method per RFC 2504.#53533

Merged

bors merged 3 commits intorust-lang:masterfrom withoutboats:error-source

Sep 1, 2018

+66-1

Commits on Aug 20, 2018

Add Error::source method per RFC 2504.
Without Boats
committed
Fix feature name.
Without Boats
committed
Correctly parenthesize dyn Error + 'static.
Without Boats
committed