Add class Bit_addr

To reduce the number of parameters in Type_handler methods (such as make_table_field_from_def(), see MDEV-9216), we'll introduce a new class representing the position and the offset of a bit.
It will be reused for:

• NULL bits (for NULL-able columns)
• Data bits (for the BIT(N) data type, and similar)

The tentative prototype:

class Bit_addr

{

  /**

    Byte where the bit is stored inside a record.

    If the corresponding Field is a NOT NULL field, this member is NULL.

  */

  uchar *m_ptr;

  /**

    Offset of the bit inside m_ptr[0], in the range 0..7.

  */

  uchar m_offs;

public:

  Bit_addr(uchar *ptr, uchar offs)

   :m_ptr(ptr), m_offs(offs)

  {

    DBUG_ASSERT(ptr || offs == 0);

    DBUG_ASSERT(offs < 8);

  }

  uchar *ptr() const { return m_ptr; }

  uchar offs() const { return m_offs; }

  uchar bit() const { return m_ptr ? ((uchar) 1) << m_offs : 0; }

};

Encapsulation is needed for better consistency control, to make sure that m_ptr and m_offs always represent valid combinations.

To further reuse the code, we'll replace Record_addr members null_ptr and null_bit to an instance of Bit_addr.