skip updates where possible

[Rich-Harris]

This will fix #768. With #760 in mind, I'm going beyond just skipping tag updates, and changing how updates work at a slightly more fundamental level. _set is now a choke point where all the differs checks take place, and rather than passing round the new state object, we pass around a changed object that tracks which keys are actually dirty. This can be used by dispatchObservers, recompute and so on.

_set is now also part of the shared prototype. The only thing that changed from one component to the next is recompute, so that's now a prototype method rather than a standalone function. This will result in byte savings in apps with multiple components.

TODO:

• apply same thing to raw tags, and attributes
• don't bother caching values of text nodes etc where it's unnecessary. we can just use the changed check

[Rich-Harris]

Taking the WIP tag off. This makes a huge performance difference on the app I'm currently working on