Implementing dedicated fixing methods:
- Type_handler_bit::Item_func_round_fix_length_and_dec()
- Type_handler_bit::Item_func_int_val_fix_length_and_dec()
- Type_handler_typelib::Item_func_round_fix_length_and_dec()

because the inherited methods did not work well.

Fixing:
- Type_handler_typelib::Item_func_int_val_fix_length_and_dec
  It did not work well, because it used args[0]->max_length to
  calculate the result data type. In case of ENUM and SET it was
  not correct, because in FLOOR() and CEILING() context
  ENUM and SET return not more than 5 digits (65535 is the biggest
  possible value).

Misc:
- Changing the API of
    Type_handler_bit::Bit_decimal_notation_int_digits(const Item *item)
  to a more generic form:
    Type_handler_bit::Bit_decimal_notation_int_digits_by_nbits(uint nbits)

- Fixing Type_handler_bit::Bit_decimal_notation_int_digits_by_nbits() to
  return the exact number of decimal digits for all nbits 1..64.
  The old implementation was approximate.
  This change gives better (more precise) data types.