Implement Element.prefix (Fixes #1737)

[ngsankha]

See #1737.

r? @Ms2ger

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @metajack (or someone else) soon.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1340

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[Ms2ger]

r+

[bors-servo]

saw approval from Ms2ger
at ngsankha@36bf23d

[bors-servo]

merging sankha93/servo/prefix-impl = 36bf23d into auto

[bors-servo]

sankha93/servo/prefix-impl = 36bf23d merged ok, testing candidate = c760577

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/1257
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/1211

[bors-servo]

fast-forwarding master to auto = c760577