HTML API: Active format reconstruction with noah's ark

[sirreal]

WORK IN PROGRESS

Implement Noah's Ark when pushing to the stack of active formatting elements.

If there are already three elements in the list of active formatting elements after the last marker, if any, or anywhere in the list if there are no markers, that have the same tag name, namespace, and attributes as element, then remove the earliest such element from the list of active formatting elements. For these purposes, the attributes must be compared as they were when the elements were created by the parser; two elements have the same attributes if all their parsed attributes can be paired such that the two attributes in each pair have identical names, namespaces, and values (the order of the attributes does not matter).

TODO:

• When adding the equivalent element, the earliest should be removed (not skip the 4th). For practical purposes in the HTML API, this may be irrelevant and can be ignored.
• Are the performance characteristics of this acceptable?
• Fix all the lints and improve the new classes and constructors.

 FAILURES!
-Tests: 1494, Assertions: 1076, Failures: 2, Skipped: 418.
+Tests: 1494, Assertions: 1097, Failures: 2, Skipped: 397.

Builds on WordPress#6982.

Trac ticket:

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