Add oci8_allocate_typeddata() and use it to allocate OCI objects.
Rename vtable and vptr to data_type.
Use rb_data_type_t as the base type of oci8_handle_data_type_t.
Add more write barriers.
Add check_oradate() to check datatype.
Add oci8_check_typeddata() and use it instead of oci8_get_handle() and oci8_get_bind().
Revise type checking of LOB.