Implement ParentNode.children for Document and Element

[evilpie]

Fixes #2186

[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/1329

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[Ms2ger]

r+

[bors-servo]

saw approval from Ms2ger
at evilpie@4c057de

[bors-servo]

merging evilpie/servo/children = 4c057de into auto

[bors-servo]

evilpie/servo/children = 4c057de merged ok, testing candidate = b35d830

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/1264
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/1218

[bors-servo]

fast-forwarding master to auto = b35d830