Native record

native record primitives are defined in (picrin record-primitive)
- (make-record rectype)
- (record-of? record rectype)
- (record-ref record rectype field-name)
- (record-set! record rectype field-name value)

define-record-type is implemented using these primitives.
Curretly, record-type is also record-object. Its record-type is #t, and have only name field.
@zeptometer 

I see what you want to do, but it should be checked in `define-record-accessor`, not in `record-ref`, shouldn't it? Even if `(get-slot1 b)` is an error, I expect `(record-ref b 'slot)` return 4 in this example.