For Array#sample and Array#shuffle. They used to take optional ordinal
argument for Random object but CRuby uses `random:` keyword argument.
Now mruby is compatible with CRuby here.
* The `Fixnum` constant is now an alias for the `Integer` class.
* Remove `struct mrb_state::fixnum_class` member.
If necessary, use `struct mrb_state::integer_class` instead.
The ISO standard does not include implicit type conversion using
`to_int`. This implicit conversion often causes vulnerability.
There will be no more attacks like #4120.
In addition, we have added internal convenience method `__to_int` which
does type check and conversion (from floats).
Yukihiro "Matz" Matsumoto