Added muFailReturn to muRascal and FAILRETURN to RVM

- reorganized RVM AST and documented every instruction
- a Rascal fail statement outside a backtracking context now generates a
muFailReturn which ends up as FAILRETURN.
Added more tests for expressions
Added and implemented FAILRETURN in RVM

Note: FAILRETURN now behaves like RETURN0, but this should change to a
return to the overloading resolution frame.