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).

The ISO standard does not include implicit type conversion using
`to_int`, `to_str` and sometimes `to_f`. For the compactness of the
mruby implementation, maybe we should remove those implicit conversion
from mruby.

`mrb_raisef()` only takes `%S` specifier. If you don't have extra
arguments, use `mrb_raise()`.

The fix was proposed by https://hackerone.com/aerodudrizzt

The issue (and the fix) reported by https://hackerone.com/aerodudrizzt