Compile with -Weverything flag

[abbeycode]

This produces a ton of warnings, but the problem is I don't want any warnings reported for the unrar library. I can solve this by moving the unrar library into its own static library that'll get compiled in, I think. Then I'll specify -Weverything for UnrarKit, and -Wnone for unrar. This makes sense after #55, since it involves changing target configuration around.

• Split unrar into its own library, linked into UnrarKit
• Specify -Weverything for UnrarKit
• Fix all warnings produced

[aonez]

Those improvements are so cool (the libunrar specifically) 👍🏼