support ORM bulk into

type_holder support implicit convert
add typename in type-conversion.h, do_use
fpr pass common test test12
edit convertible class. change strategy of implicit cast
fix a Oracle backend bug:

create table T(t1 number);

for column t1, oracle_statement_backend::describe_column(...) return dt_integer, should return dt_double
add test_convert()

add test_orm_bulk()