Negative impls integrated into coherence

[nikomatsakis]

Summary

Introduce explicit negative impls, like impl !Error for str, that are integrated into coherence. These impls extend the overlap rules.

Info

• Owner: @nikomatsakis
• Liaison: @pnkfelix
• Implementor: @spastorino
• Zulip stream #wg-traits
• Stage: Experimental
• Repository rust-lang/negative-impls-initiative (rendered)

What is this issue?

This issue represents an active initiative. It is meant to be used for
the group to post updates to the lang team (and others) in a lightweight
fashion. Please do not use the comments here for discussion, that should be kept
in the Zulip stream (discussion comments here will be marked as off-topic).