Support MathML and pass ACID MathML Test Suite

[innovimax]

Support MathML in servo and also pass ACID MAthML Test Suite
http://fred-wang.github.io/AcidTestsMathML/acid1/
http://fred-wang.github.io/AcidTestsMathML/acid2/
http://fred-wang.github.io/AcidTestsMathML/acid3/

[runarberg]

Sorry, I didn't mean to link to this issue. I'm sort of doing this as a hobby to see how far I'm able to get plus learning a little how the DOM and browser layout is implemented. If I make any progress in the future I might send a PR.

[jdm]

Cool! Let us know if you have any questions :)

[fred-wang]

@runarberg Thanks for working on this. I'm willing to discuss this with you in more details later, but for now I think that this document will be helpful: http://www.mathml-association.org/MathMLinHTML5

[pshaughn]

WPT cares about parsing MathML inside HTML and vice versa: html/syntax/parsing/html_content_in_foreign_context.html wants to know that if an HTML (non-MathML) element is embedded inside <math>, its parent node will be the parent of the <math>. OTOH, all browsers are failing this test at some point between the math part and the svg part, so not a high priority here.