* Separate LoadPtr IR instructions into pass1 and pass2 variants.
* Define `type_size_bits` for extern structs to be the same as
their `@sizeOf(T) * 8` and allow them in packed structs.
* More helpful error messages when trying to use types in
packed structs that are not allowed.
* Support arrays in packed structs even when they are not
byte-aligned.
* Add compile error for using arrays in packed structs when the
padding bits would be problematic. This is necessary since
we do not have packed arrays.
closes#677
1066004b79