Rewrite HaLVM3 base library to stop using 'unsafe' so many places

[acw]

By calling back into GHC for a bunch of critical POSIX functions, we break the "unsafe" assumption in the base library. So HaLVM GHCs will need to revoke those unsafe declarations. In particular, the ones in System/Posix/Internals.hs definitely need to be re-evaluated.