fix is_numeric/is_integer when checking non-string parameters

I expect a function called "is_numeric" or "is_integer" to check if a
variable is an integer or a number even if the variable passed by isn't
a string nor a number at all. Otherwise we should call them
is_string_a_number and is_string_an_integer and we have then to remove
the check for .is_a?(Number) and .is_a?(FixNum)
now checking also if it is a hex or octal number
improved/corrected checking for integer

* checking against Integer instead of Fixnum so that
  also Bignum is matching
* now .is_a? Integer is done first so this is quiet fast
Now many types of numerics are recognized.

1. Float/Integer values (signed or unsigned, with exponent or without)
2. octal and hex check
3. except hex numbers and the "0." in a float lower than 1 can be prefixed
   with a '0'.