Implement DOMTokenList.{add,remove,toggle}

[brunoabinader]

Specs:
https://dom.spec.whatwg.org/#dom-domtokenlist-add
https://dom.spec.whatwg.org/#dom-domtokenlist-remove
https://dom.spec.whatwg.org/#dom-domtokenlist-toggle
https://dom.spec.whatwg.org/#concept-dtl-update
https://dom.spec.whatwg.org/#concept-ordered-set-serializer

Closes #3138.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3486

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[bors-servo]

saw approval from Ms2ger
at https://github.com/brunoabinader/servo/commit/3624673d2faf7a36face3321af70fc55117a24a7

[bors-servo]

merging brunoabinader/servo/domtokenlist = 3624673 into auto

[bors-servo]

brunoabinader/servo/domtokenlist = 3624673 merged ok, testing candidate = 3af73e9

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/410
success: http://build.servo.org/builders/linux2/builds/409
success: http://build.servo.org/builders/android/builds/445
success: http://build.servo.org/builders/mac1/builds/626
success: http://build.servo.org/builders/mac2/builds/627

[bors-servo]

fast-forwarding master to auto = 3af73e9

[hmac]

Fixes most of #2973