Because they're defined in both `mruby-string-ext` and `mruby-numeric-ext`
(they seem more natural to define in N, but `mruby-string-ext` depends on
`Integral#chr`).
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).
- reduce compile time by a little bit (full-core: ~0.7s for me)
- thanks to 'include-what-you-use' for some help
- include Standard C header files before any other (coding style)
Yukihiro "Matz" Matsumoto