Don't wrap str pointer in Packed

[jplatte]

Closes #9.

[zzorba]

It's a shame this has never been merged, as the library remains unlinkable on iOS without it.

[rodrimati1992]

I'll merge this when I see evidence that this behavior with packed pointers is acknowledged as being either:

• a compiler bug
• an unfixable incompatibility with the linker (unfixable in the compiler)