The SliceFlag implementation and associated aliases (MultiStringFlag, etc)
extend the existing slice implementations (StringSliceFlag, etc) to support
actual slices as the flag value and destination.
This change also fixes various bugs in the existing implementation. Notably,
the StringSliceFlag.Apply implementation would modify the input (default)
Value, if an env var was set, and no destination was provided. The bugs fixed
in the other three implementations were all already fixed in either
StringSliceFlag, or in one case (ignoring empty env var) in Float64SliceFlag.
