further work on ref counting
added garbage collector tests
also corrected error in fibo.scm
removing some now redundant const-correctness

our language will have mutability so it doesn't make sense for our hash
to store const values
updating TODO with all places that must be changed to deal with refcounting
top level eval now handles ref count decr
define form requires no changes
added commented out line to deal with if form
parsing now uses plot_value_new
updating unit tests to account for parse using plot_value_new and expr containing a plot_value * (rather than plot_value)