Add a helper class Record_addr

These three parameters:

  uchar *ptr_arg,

  uchar *null_ptr_arg,

  uchar null_bit_arg

are always passed to Field_xxx constructor.

Under terms of MDEV-4912 we'll introduce a few methods in Type_handler which will do "new Field_xxx". Before doing this, to make the Type_handler API look simpler, we'll introduce a new class:

Record_addr

{

public:

  uchar         *ptr;                   // Position to field in record

  /**

     Byte where the @c NULL bit is stored inside a record. If this Field is a

     @c NOT @c NULL field, this member is @c NULL.

  */

  uchar         *null_ptr;

  uchar         null_bit;               // Bit used to test null bit

};

Then:

• Derive Field from Record_addr
• Change Field constructors to accept a "const Record_addr &addr" parameter instead of the three separate parameters ptr_arg, null_ptr_arg, null_bit_arg.