Split Item_equal::add_const() into a virtual method in type_handler()

The method Item_equal::add_const() has this code:

switch (Item_equal::compare_type_handler()->cmp_type()) {

This is not friendly for user data type plugins, because user data types will need their own value comparison implementation, different from the ones for the built-in data types.

This task is to split this swith into a new virtual method in Type_handler with approximately this API:

virtual bool Item_eq_value(THD *thd, Item *a, Item *b) const;