Support stmt blocks

Later we will have to deal with variable aliasing issues that support of
stmt blocks introduces. For example, the following YCP code:

  {
    integer i = 5;
    {
      integer i = 6;
    }

    // i == 5 here
  }

currently gets translated into wrong Ruby code:

  i = 5
  i = 6
  # i == 6 here
Implement the "break" statement