Rename the JIT function Default() to FallBackToInterpreter(). Communicates intent way better in terms of telling the reader what's going on.

Also did some minor cleanup in functions that used Default.
Merge pull request #122 from lioncash/rename-default

Rename the JIT function Default() to FallBackToInterpreter(). Communicates intent way better in terms of telling the reader what's going on.