Opt-in builtin traits, take 2: default and negative impls.

The high-level idea is to add language features that simultaneously
achieve three goals:

1. move `Send` and `Share` out of the language entirely and into the
   standard library, providing mechanisms for end users to easily
   implement and use similar "marker" traits of their own devising;
2. make "normal" Rust types sendable and sharable by default, without
   the need for explicit opt-in; and,
3. continue to require "unsafe" Rust types (those that manipulate
   unsafe pointers or implement special abstractions) to "opt-in" to
   sendability and sharability with an unsafe declaration.

Maybe `_` is better? `impl Trait for _ {}`