Auto merge of #12469 - emilio:stylo, r=bholley

style: Rewrite the restyle hints code to allow different kinds of element snapshots.

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This is a rewrite for how style interfaces with its consumers in order to allow
different representations for an element snapshot.

This also changes the requirements of an element snapshot, requiring them to
only implement MatchAttr, instead of MatchAttrGeneric. This is important for
stylo since implementing MatchAttrGeneric is way more difficult for us given the
atom limitations. This also allows for more performant implementations in the
Gecko side of things.

I don't want to get this merged just yet, mainly because the stylo part is not
implemented, but I'd like early feedback from @bholley and/or @heycam: How do
you see this approach? I don't think we'll have much problem to implement
MatchAttr for our element snapshots, but... worth checking.

r? @heycam

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