Update css-tree [WIP]

[Conduitry]

csstree/csstree#47

This has apparently been addressed, and it looks like we can now import just the parser. (It looks like at some point between then and now we stopped using css-tree's walker and started using estree-walker for walking the AST?) This upgrade does increase the bundle size, but not as dramatically as alpha16 -> alpha17 would have.

I had to make one other change for a renamed property in order to get the tests to pass, but css-tree's changelog should be looked at more thoroughly.

[Conduitry]

Ref #408.

Also, the failed CI tests looked unrelated. Rebasing on latest master and hopefully that clears them up.

Edit: That ... did not help. I think we're good now

[Rich-Harris]

Nice! this is great news

[lahmatiy]

@Conduitry Do you count the difference in size?
I experimented with improvements on bundling of separate modules (parser for the beginning). Work still in progress (on pause at the moment), but I got ~35Kb for parser instead of ~58Kb (current size of build by browserify). I think the result can be improved. Anyway, for smaller size you will need to use a build version. Is it an option for you? Does further size reducing make sense for you?
//cc @Rich-Harris