context-specific namespace references
-------------------------------------

Context-specific namespace references are looked up in the namespace of the
type where they are used. Here are some example uses:

Common values for types and "defaults":

    typedef PageSize = (float w h);
    
    PageSize PageSize:A4 = (210, 297);
    PageSize PageSize:USLetter = (215.9, 279.4);
    PageSize:_ = :A4;
    
    PageSize pg = :_; // _ is the "default"


Enumerations values:

    typedef bool = (false=0, true=1);
    
    bool a = :false;

    () f(bool x);
    
    f(:true);


TODO: How should default parameters be implemented? Can a special form of
      context-specific parameters be used? For example:
      
      () f(bool x);
      meta f:x:_ = :false;
      
      f(._); // access to f:x:_
      
      // Note: We can access stuff outside the designated namespace!
      //       Is this a good idea?
      
      
      Maybe it's better to have a special syntax:
      
      () f(bool x=:false);
      
      f(_);