Draft: add next-generation error trait

Rust's standard library's error trait leaves a lot to be desired, especially with regards to handling groups of errors and transparent wrappers over other errors. This is an attempt to fix that.

To-do list:

• New error trait.
• Error forest traversal.
• Put core and ng modules behind feature flags. (Don't forget to test feature combinations.)
  • Too annoying, not doing this.
• Decide whether Errors should be cloneable (via dyn-clone).
  • I've dropped dyn-clone because it currently requires alloc rather than just core. Maybe a core-only dyn-clone is possible since the Clone trait is actually in core, but I dunno. Anyway, it would be really nice if this new error trait was core-only, which means dyn-clone has to go away.
• Add impl<E: Error + ?Sized> Error for SmartPointer<E> impls.
• Add a compatibility newtype wrapper for core::error::Error.
• Convenience functions for formatting/printing an error forest.
• Support downcasting.
• Procedural macro for implementing Error on structs and enums.
• More?