An trivial support for detecting unbound variable
Add a trivial support for input stream made from string
backup
Add the variable `global_env' for representing the real global environment
Remove the uses of reference-count garbage collection
Remove the field `prev' in type `struct lisp_object_t'
Implements a basic mark-sweep GC without testing
Fix a bug when scanning the heap and sweeping

The collector should just work on the object which is used but not be marked
Merge branch 'feature-mark-sweep-gc' into develop
Removes unnecessary comments and update the version