Option<NonNullablePtr> optimization is unavailable to library smart pointers

[thestinger]

[sanxiyn]

Triaging. See #8678 for what happened since the last time.

[nikomatsakis]

I think there is general agreement that we need some sort of attribute smart pointers can use to opt-in here.

[nikomatsakis]

An RFC seems appropriate.

[emberian]

I can write up an RFC for #[unsafe_not_null] today.

[emberian]

And by today I mean "much later"

[emberian]

rust-lang/rfcs#36