This PR does some of the work to switch from Jimfs to memoryfilesystem.

This has failing tests:

• JimfsFileSystemProviderImplTest.theCreatedFileSystemSupportsDirectoryStreams:166 » IllegalState #iterator() already called. DirectoryStream#iterator() is specified to throw IllegalStateException if the iterator has already been returned
• JarFactoryImplTest.exceptionRaisedIfOutputJarDirectoryDoesNotExist:76

Expecting message to be:
  "/some-non-existent-directory/HelloWorld.jar"
but was:
  "/some-non-existent-directory"

assertions on exception messages are quite brittle. It would probably be safer to use java.nio.file.FileSystemException#getFile()

• FileUtilsTest.retrieveRequiredUrlCanDereferencePathsToUrls:89

Expecting actual:
  "memory:45377dad-eb8b-42ac-96df-5d3d9f18e5c9:///foo/bar/baz.txt"
to start with:
  "memory://"

The JimfsFileSystemProviderImpl* classes should probably be renamed.

I do not plan to put any more work into this PR.