Fix alignment issue in md_mark_store_ptr()

[wmww]

Fixes #289 by making MD_MARK_tag a union (so the compiler will give it the correct alignment). Since the main member of the union is an anonymous struct, its properties can still be accessed as if they're toplevel MD_MARK_tag properties.

[N-R-K]

Both the store and load are done via memcpy - which works at byte level - and thus there's no alignment issues. Whatever tool you're using is giving false positive warning.

[wmww]

I would expect it to be well defined, I suppose a false positive is a possibility. In any case I think my patch (especially now after edits) makes the code clearer, simpler and shorter as well as making it play nice with Fil-C, so maybe still worth merging.