Implement TreeWalker

[ChrisParis]

Addresses #1234. This implementation allows NodeFilters to be provided as JavaScript objects, for the normal use case, but also as native Rust functions. This is so that TreeWalker can be used internally. This could be useful, for example, in implementing HTMLOptionElement.text, in which we have to skip script elements and children. You can see this worked out at ChrisParis@c1bb1e7.

I've submitted test cases to https://github.com/w3c/web-platform-tests. The tests in which the NodeFilter either isn't callable or throws an exception don't work. It looks like that has something to do with the automatically generated code.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2536

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[jdm]

Woo! This is an exciting PR! 🐨

[jdm]

/dom/interfaces.html
--------------------
PASS expected FAIL Document interface: xmlDoc must inherit property "createTreeWalker" with the proper type (23)
PASS expected FAIL Document interface: calling createTreeWalker(Node,unsigned long,NodeFilter) on xmlDoc with too few arguments must throw TypeError
PASS expected FAIL TreeWalker interface: attribute root
PASS expected FAIL TreeWalker interface: attribute whatToShow
PASS expected FAIL TreeWalker interface: attribute filter
PASS expected FAIL TreeWalker interface: attribute currentNode
PASS expected FAIL TreeWalker interface: operation parentNode()
PASS expected FAIL TreeWalker interface: operation firstChild()
PASS expected FAIL TreeWalker interface: operation lastChild()
PASS expected FAIL TreeWalker interface: operation previousSibling()
PASS expected FAIL TreeWalker interface: operation nextSibling()
PASS expected FAIL TreeWalker interface: operation previousNode()
PASS expected FAIL TreeWalker interface: operation nextNode()

[ChrisParis]

PASS expected FAIL --
My favorite kind of bug.

[jdm]

You're correct that the test failures fall outside the purview of this PR. This is ready to merge after rebasing and squashing!