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.
Yukihiro "Matz" Matsumoto