"Owned" should imply "Durable"

[bstrie]

See @nikomatsakis' comments on #4446.

[graydon]

assigning bug; change assignment if you disagree

[brson]

Bumping from 0.7

[thestinger]

The Durable kind no longer exists.

[nikomatsakis]

Untrue, it is just called 'static now.

[catamorphism]

From eyeballing middle/ty.rs, this looks true to me. I'll check in a test.