Implement live HTMLCollection cache for Document.links

[brunoabinader]

Closes #1885.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2292

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[jdm]

ERROR:script::html::hubbub_html_parser: error loading script file:///Users/travis/build/servo/servo/src/test/content/src/test/content/harness.js

ERROR:js::rust: Error at file:///Users/travis/build/servo/servo/src/test/content/test_document_links_cache.html:8: ReferenceError: is_not is not defined