[Fix #552] Load resources from jar/zip files efficiently.

Instead of loading the entire `.jar` file into a buffer, just extract the single resource to be displayed. Mark it read-only. If that resource is already open, reuse it. This eliminates the overhead for `jump`-ing into jar/zip files.

Since we no longer potentially load large files just by navigating code, it seems reasonable to allow loading `tramp` resources to be the default.