Convert a positive number string to a Num.
parsePositive "123"
parsePositive {a=Int} " +123"
Convert a number string to a Num.
parseInteger " 123"
parseInteger {a=Int} " -123"
Convert a number string to a Double.
parseDouble "+123.123e-2"
parseDouble {a=Int} " -123.123E+2"
parseDouble {a=Int} " +123.123"