Major changes:
- `compiler.compileRoot(el, type)`
-> `compiler.compileInHost(type) + viewHydrator.hydrateHostViewInPlace(el, view)`
- move all `hydrate`/`dehydrate` methods out of `View` and `ViewContainer` into
a standalone class `view_hydrator` as private methods and provide new public
methods dedicated to the individual use cases.
Note: This PR does not change the current functionality, only moves it
into different places.
See design discussion in #1351, in preparation for imperative views.
