Switched back to string pointers for optional values.
Switched back to string pointers for optional values.

It turns out that using string pointers can actually be done reasonably
elegantly, so this commit moves nullableString and just adds a simple
stringptr function to avoid capturing pointers to loop variables.

Signed-off-by: Jacob Howard <f862f167b85d41b225785c70d70808bc7337c1fe@havoc.io>

havoc-io/mutagen,havoc-io/mutagen