"server side rendering" is a misleading term since it implies there is a
server. that isn't neccecarily the case here, since it supports running
in the browser. I think "clover engine" is cute, short for "clover html
rendering engine". Instead of "server side rendering", it's just rendering.
This commit makes things a lot more concise, such as `ssr.ssrAsync`
being renamed to `render.async` to play nicely with namespaced imports.
`getCurrentRender` and `setCurrentRender` are just `current` and
`setCurrent`, and the addon interface has been redesigned to force
symbols with a wrapping helper.
