Modify lval_print to print function names
	Modify lval_fun constructor to set sym field to function name
	Modify lval_copy to copy sym field for lval_fun
Modify lval_print to print functions as <function: function_name>
builtin_vars function for printing all symbols in environment
	Currently requires calling with an argument, change?
Modify lenv to track reserved symbols
	Modify lenv_new and lenv_del to init/cleanup reserved symbol list
	Add lenv_put_reserved function to add to reserved symbol list
	Add builtin_reserved function to print all reserved symbols
Merge branch 'master' into 11
Add support for calling functions with no arguments
	Modify lval_fun to store expected argcount in lval_fun->count
	lval_fun constructor takes int argcount argument
	lval_eval checks function argcount and calls if argcount == 0
Add ability to exit REPL by typing "exit"
	builtin_exit function, add "exit" to builtin list
Print functions as <function 'function_name'>