Allexe: Redesign to avoid write-on-destructor behavior, fix buggy writing behavior

[dtzWill]

• Errors during write aren't available for handling/printing/checking
• No (clean) way to exit program with attempting to write the Allexe contents to disk (!!)

I think this RAII-style behavior is at odds with the desired behavior, although if there's
a way to make it work then by all means :). But also libzip doesa fair bit of magic that
is probably more confusing/problematic than we want at the moment...

On that, our read/write paths could be a lot simpler and clearer...
current design makes for workarounds resulting in things like (old issues 58 and 59).

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Actually, it might be more of an ZipArchive problem than Allexe-- closing the zip on destruction makes sense if it's like a file descriptor, but instead libzip basically defers everything to that point.

That still leaves the error handling of closing the FD but that's much less critical than what we currently have.

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Another write behavior we can't handle properly: unable to create output file because directory isn't +w.