Api and internals improvement

Features

• Easier setup using only C++
• VNodes are now normalized only when needed, improving performance

Fixes

• refs are now correctly called with null on callback change

Breaking changes

• No need to import 'asm-dom/cpp/' (or asm-dom.js) in javascript to prepare asm-dom and getting started

• toHTML API in C++ changes its signature as follow

   // before
   // std::string toHTML(const VNode* const vnode);
  
   // now
   std::string toHTML(VNode* const vnode);