Abstract away RamFileSystem common logic that works for any file system and make alternative to ramdisks

[ascopes]

To assist in debugging in the future, and to potentially improve functionality with compilers like ECJ that have a dependency that the file system is the default file system internally, move most of the logic for RamFileSystem to an abstract class, and make RamFileSystem extend this abstract class.

Then implement an alternative to this that uses temporary directories on the default file system.