firebase-ios-sdk

Migrate LevelDB data out of ~/Documents (#4609)

This moves iOS and macOS user data from `NSDocumentDirectory` to `NSApplicationSupportDirectory`, adding a pre-open migration step that:

  * Moves old data into the new location
  * Cleans up any resulting empty directories in the old Documents folder
  * Stops before removing the Documents folder itself (see tests)

Note that other platforms don't need a migration:

  * tvOS was already storing documents in `NSCachesDirectory`
  * Linux, Windows, and other UNIX haven't been publicly released or aren't changing.

See individual commits for details. Most notably this adds some additional filesystem manipulation routines to our (sadly) growing library.

Fixes #843.