I want to get the offset of fields inside structs, but I don't want to
instantiate the struct. I need to embed the offsets inside machine
code, and I can't get the offsets without calling `new` on the struct.
This commit adds an `offset` method so you can get the offset of a
member without instantiating anything. You can do:
```ruby
C.rb_control_frame_t.offsetof(:sp) #=> 8
```
I don't think this implementation is perfect, you can only get immediate
fields. But it is better than nothing!