Lifetime all the type aliases for Handle and MutableHandle

[marmistrz]

That's a follow-up to https://github.com/servo/servo/pull/20246

Especially:

• https://github.com/servo/servo/pull/20246#discussion_r177792108
• https://github.com/servo/servo/pull/20246#discussion_r177791615

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This change is 

[nox]

@bors-servo r+

Great!

[bors-servo]

:pushpin: Commit acd776f has been approved by nox

[bors-servo]

:hourglass: Testing commit acd776f1940b8a6fc94b30fada1c061902945283 with merge 5b79f789520d66a5391033bb78ac30b81e43d8b1...

[bors-servo]

:sunny: Test successful - status-appveyor, status-travis Approved by: nox Pushing 5b79f789520d66a5391033bb78ac30b81e43d8b1 to master...