[webcomponents] Composition Algorithm shouldn't unwrap slots (#308)

[Composition Algorithm](https://w3c.github.io/webcomponents/spec/shadow/#composition-algorithm) defined in section 4.1 "unwraps" slots by re-distributing distributed nodes of a distributed slot by invoking [Distributed Nodes Algorithm](https://w3c.github.io/webcomponents/spec/shadow/#dfn-distributed-nodes) in section 3.3.

However, I don't think that should happen.  In the world where re-distribution is explicit, there is no point in stripping out slot elements.  It's better to treat a slot element like any other element and re-distribute it on its entirely.  This will make the distribution/composition algorithm much simpler and allows slot element to be styled properly.

@hober @othermaciej @annevk @travisleithead 

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/308