renamed FileImpl -> StoredFile
style fixes in DiskFileSystem
added FileSystem.root()
refactored StoredFile to use SubFileSystem
refactored StoredFileSet to use SubFileSystem
removed StoredFileSet.root()
refactored SandboxImpl
refactored FileFunction
remove not used method from TestingSandbox
removed one of StoredFile constructors, refactored code that uses it
split File into File and MutableFile
split StoredFile into StoredFile and MutableStoredFile
fixed StoredFileSet.file(Path) to throw exception when no file at given path exists
added TestingFileSystem.createPathContainingItsPath(Path)
refactored StoredFileSet
added missing test to StoredFileSetTest
moved StoredFileTest.assertContentHasFilePath() to TestingFile