better error handling and reporting
eval: fix conversion of numbers to words
eval: fix conversion between words and numbers
prim: split off LogoControl from LogoPrim
control: add 'catch', 'throw'
control: move exceptions to LogoControl
control: add 'pause', 'continue'
control: add 'ignore'
rename LogoEval.execute -> LogoEval.instructionlist
control: add 'do.while'
control: add 'while', 'do.until', 'until'
arith: add 'random', 'rerandom'
arith: add 'form'
arith: add 'bitand', 'bitor', 'bitxor', 'bitnot'
prim: add 'lessp', 'greaterp', 'lessequalp', 'greaterequalp'
prim: rearranged LogoPrim
prim: added 'equalp' & frieds, 'numberp', 'count'
introduced a way to specify argument restrictions
prim: add 'definedp', 'namep' + improved arg gadt
graphics: add 'arc', 'pos', 'xcor', 'ycor', 'heading'

'arc' is only implemented for the Graphics backend for now.