refactored PrecalculatedTask to not extend AbstractTask
renamed Task.calculateResult() -> execute()
style fixes in Task/AbstractTask
refactored FileSet to implement Iterable<File>
renamed FileSetImpl -> StoredFileSet
removed unused ImmutableFileSet
added Sandbox.createFile()
added MutableFileSet
redesigned buiting functions to use Sandbox.createFile() + MutableFileSet instead of Sandbox.createResultFile()/createResultFiles()
removed Sandbox.resultFile()/resultFileSet()
removed no longer needed FileSet.createFile()
name cleanup in FileSystem