Add trait object field types to back/abi.rs, and use them

I've added trt_field_vtable, trt_field_box, and trt_field_tydesc, and
inserted them in place of the "magic numbers" used to access trait
object fields through GEPi().
auto merge of #6172 : Sodel-the-Vociferous/rust/rm_trt_obj_magic_nums, r=catamorphism,graydon

I don't know how one would write a separate test for this sort of thing. Building the compiler, and `make check` worked, which should mean I didn't screw anything.