Consider to support access specifiers (at least for functions)

[AntonSulimenkoHarman]

Motivation: Cast in zs expressions #510

Simplified example:

struct Fraction
{
  uint8 num;
  uint8 den;
  
  private function float32 numAsFloat32()
  {
    return num;
  }
  
  function float32 resultHack2()
  {
    return numAsFloat32() / den;
  }
};