API cleanup

Frontend refactoring. Most notably, this enables multiple medea contexts on a page without interferences. 

 - Moved all static parts to `medealib` (most notably debug logging, assert, `Class` ..)
 - `medealib.CreateContext` replaces `medea.Ready` and receives as parameter to the callback a `medealib.Context`.
 - `medealib.Context` offers the same API as `medea` did before globally. 
 - module execution is no longer delayed until the first API from a module is called. `_initMod()` is therefore gone, and so are the problems with constants not being available prior to calling an API.

Samples are not updated yet (except `Terrain`).