Add Type_handler::Item_val_bool()

The code in this method:

bool Item::val_bool();

is not friendly to pluggable data types, as it tests for result_type().
Pluggable data types may want to implement a different way for boolean conversion.

Under terms of this task will add new methods in Type_handler:

virtual bool Item_val_bool(Item *item) const;

and split the mentioned code into virtual implementations for relevant Type_handler_xxx classes.