Optimize controls to RawLiterals at compile time

[exyi]

It should be possible to render simple controls at compile time to simple literals. We should implement this at least for HtmlGenericControl and have interface where anyone could connect their controls (or maybe use magic like symbolic execution to figure out what the control renders ;) ). This optimization can be bit dangerous if some control relies on what in their content is. I'd leave first level descendants and path to root untouched for "blackbox" controls by default, it could be overridden by an attribute on the control and by some extensibility interface. We could also keep a lazy lading template in case the controls are needed later at runtime.

We also discussed this in #100, but the point was slightly different.