Proof of concept: parameterized node traversal methods.

[frewsxcv]

This change is 

[highfive]

Heads up! This PR modifies the following files:

• @fitzgen: components/script/dom/htmltitleelement.rs, components/script/dom/document.rs, components/script/dom/node.rs, components/script/dom/documentfragment.rs, components/script/dom/htmlfieldsetelement.rs, components/script/dom/characterdata.rs, components/script/dom/servoparser/html.rs, components/script/dom/htmloptgroupelement.rs, components/script/dom/servoparser/mod.rs, components/script/dom/htmltableelement.rs, components/script/dom/element.rs, components/script/dom/htmltablecellelement.rs, components/script/dom/htmloptionelement.rs, components/script/devtools.rs
• @KiChjang: components/script/dom/htmltitleelement.rs, components/script/dom/document.rs, components/script/dom/node.rs, components/script/dom/documentfragment.rs, components/script/dom/htmlfieldsetelement.rs, components/script/dom/characterdata.rs, components/script/dom/servoparser/html.rs, components/script/dom/htmloptgroupelement.rs, components/script/dom/servoparser/mod.rs, components/script/dom/htmltableelement.rs, components/script/dom/element.rs, components/script/dom/htmltablecellelement.rs, components/script/dom/htmloptionelement.rs, components/script/devtools.rs

[highfive]

Warning

• These commits modify script code, but no tests are modified. Please consider adding a test!

[frewsxcv]

I've had this sitting around in a branch for a while, thought I might as well open a PR. There's still a little more work to do here, but I didn't want to spend any more time on it if the idea is not well received.

[jdm]

I'm in favour!

[Ms2ger]

Can we use default type parameters yet?

[frewsxcv]

Can we use default type parameters yet?

I thought about making it ....<T=Node>(..), but then I ran into rust-lang/rust#36887

[frewsxcv]

More info: https://internals.rust-lang.org/t/interaction-of-user-defined-and-integral-fallbacks-with-inference/2496

[jdm]

Please remove this. We should only scope it for the code that requires it.

[frewsxcv]

What? This isn't a wanted improvement? :)

But actually, this is just temporary until I can figure out why the lint is complaining about the iterator structure being unrooted

[bors-servo]

☔ The latest upstream changes (presumably #14549) made this pull request unmergeable. Please resolve the merge conflicts.

[wafflespeanut]

Ping?

[frewsxcv]

I don't have much time right now to finish this up. If someone else wants to, go for it :)

[jdm]

Tracking this work in #15270.