Run adopt as part of insert

[jdm]

whatwg/dom#754

[pshaughn]

This is bigger than I thought, but I think I can still do it. Challenges here:

• The concept of a document fragments' "host" element is directly exposed in ShadowRoot, but needs to actually be stored in DocumentFragment so that template elements can also use it; this makes the layout getter for it slightly more elaborate. (@jdm got me past the tricky parts of this in IRC)
• Insert code is putting a script blocker on the owner document at the start and taking a script blocker off the owner document at the end; now that adoption is happening in mid-insert the owner document isn't actually the same document in these two places, leading to a crash.
• At least one debug assertion about nodes sharing an owner doc during the insert process is false now.

[pshaughn]

^ This PR moves adopt to where the spec says to move it, and adds the host-element check, but it's not passing as many tests as it seems like it should.

[pshaughn]

whatwg/dom#819 suggests this is not going to happen, or if it does happen it will be written a totally different way.