Fix stringify() -> stringifyAsync()

[lexidor]

The old name for this method was ->stringify().
These stragglers did not have an __Override on ->stringify(), so the typechecker could not warn us about this.

Fixes:
<br/> now renders as <br> again (not <br></br>).
<style>&</style> now renders as <style>&</style> again, (not <style>&amp;</style>).

[lexidor]

I'd have expected toHTMLStringAsync() to take precedence over toStringAsync().
If something is both a node and an UnsafeRenderable, we prefer the node's toStringAsync() in
Node::renderChildAsync() is why <style> used the default "safe" renderer which escapes the contents.
In fact, UnsafeRenderable on style is ignored and not used while rendering.

Edit: updated the link to point to the state at the latest commit. Previous link pointed to master, which changed since the link was made, so the highlighted lines were wrong.