Add working Clone impl for Yoke

[sffc]

@Manishearth Can you take a look and fix this so that the test builds? I tried using YokeTraitHack but couldn't get it to behave property for Clone since it needs T to be an owned field, but I can only get a reference to T.

See #753

[coveralls]

Pull Request Test Coverage Report for Build bd3e455ac63bb1504268a3bb69dbe3d483bc0391-PR-894

• 22 of 29 (75.86%) changed or added relevant lines in 2 files are covered.
• 81 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.07%) to 74.46%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
utils/yoke/src/trait_hack.rs	0	3	0.0%
utils/yoke/src/yoke.rs	22	26	84.62%

Files with Coverage Reduction	New Missed Lines	%
utils/yoke/src/trait_hack.rs	1	0%
utils/yoke/src/yoke.rs	1	86.89%
utils/zerovec/src/map/serde.rs	1	80.33%
components/datetime/src/options/components.rs	11	77.39%
components/datetime/src/provider/helpers.rs	12	80.37%
components/datetime/src/pattern/mod.rs	20	66.94%
components/datetime/src/skeleton.rs	35	81.97%

Totals
Change from base Build 9a25738b476593d40353e19a2dd0b2c5f9ab81a6:	0.07%
Covered Lines:	9347
Relevant Lines:	12553

---

💛 - Coveralls