- Adding class Field_int as a common
  parent for Field_{longlong|long|short|medium|tiny}
- Moving store_decimal(), val_decimal(), get_date(), store_time_dec(),
  get_date(), val_bool() from Field_num to Field_int
- Adding Field_int::val_str_from_long() and reusing it in
  Field_tiny::val_str(), Field_short::val_str(), Field_medium::val_str()
  and Field_long::val_str(). This removes a good amount of duplicate code
- Adding "const" qualifier to "virtual bool Field::optimize_range()".