added built in def function
added builtin_def to environment
updated error reporting function to accept vargs and output formatted strings
updated lenv_get to output a more useful error message
upgraded assert macro to accept varargs
upgraded head function error reporting
upgraded tail function error reporting
upgraded tail function error reporting
added type lookup for use by builtin function error reporting
upgraded head function arg type error message
upgraded tail function arg type error reporting
upgraded eval function arg type error reporting