If the `Errno::EXXX` class is defined minimally when needed, it will save about 17 KiB of RAM in a 32-bit environment.
Define the singular methods `.const_defined?`, `.const_missing` and `.constants` in the `Errno` module and act as if there are classes that have not yet been defined.
However, if real classes are enumerated, for example by the following code, the effect is lost.
```ruby
Errno.constants.map { |id| Errno.const_get(id) }
```
But I believe that such codes are rarely written with the intention of being written.
In this patch, the `Errno::EXXX#initialize` method is no longer defined for further memory reduction.
Instead, it has been merged into the `SystemCallError#initialize` method.
Internal functions can only be called from within the library.
Functions listed in `mruby/internal.h` can be called from:
* core (src/*.c)
* gems (mrbgems/**/*.c)
But not from the application linked with `libmruby`.
The `mrbgems/mruby-errno/src/known_errors_def.cstub` file has been extended, so the `mrbgems/mruby-errno/src/known_errors_e2c.cstub` file is no longer needed.
ref. #2485
The `SystemCallError#to_s` method also needed to be modified, but since it has no functional difference from the `Exception#to_s` method, it will be removed.
Yukihiro "Matz" Matsumoto