The objects of this class represent multiple conjunctive equalities
in where conditions: =(f1,f2,...fn) <=> f1=f2 and f2= ... and =fn.
The objects are used to generate new possibale paths to access
the tables when executing a query.
They are also used to optimize the execution plan 
chosen by the optimizer for the query.  


sql/item.cc:
  Introduced Item_equal
sql/item.h:
  Introduced Item_equal.
  Added traverse method.
sql/item_cmpfunc.cc:
  Introduced Item_equal.
  Added traverse mehtod.
sql/item_cmpfunc.h:
  Itroduced Item_equal and Item_equal_iterator.
sql/item_func.cc:
  Added traverse method.
  Introduced Item_equal.
sql/item_func.h:
  Introduced Item_equal.
  Added traverse method.
sql/item_row.cc:
  Added traverse method.
sql/item_row.h:
  Added traverse method.
sql/item_strfunc.h:
  Added traverse method.
sql/opt_range.cc:
  Used Item_equal in range analysis.
sql/opt_sum.cc:
  Introduced Item_equal.
sql/sql_list.h:
  Added concat and disjoin methods to lists.
  Fixed remove method for lists.
sql/sql_select.cc:
  Introdiced Item_equal:
    created Item_equal;
    used Item_equal objects to generate new paths to access tables.
    used Item_equal objects to optimize the execution plan chosen by optimizer.
sql/sql_select.h:
  Introduced Item_equal.