Add support for /dev/devices

[yamaura]

Thank you for this amazing library! I found it incredibly useful. I implemented parser of /proc/devices and test.

Notes:

• The type for major has been same with the Linux kernel code.
• While BlockDeviceEntry can be disabled via CONFIG_BLOCK, but it has not been made an Option.

[eminence]

Thank you! I'm sorry for taking so long to notice and review this PR.

This looks good, only a few comments:

While BlockDeviceEntry can be disabled via CONFIG_BLOCK, but it has not been made an Option.

The API you've added here looks good, but can you update the doc comment to say that the block_devices list can be empty if the kernel doesn't support block devices?

Also, these structs have #[allow(non_snake_case)] but that doesn't seem to be needed.

[yamaura]

I've fixed the items you pointed out. Please take a look!

[eminence]

Thanks! Quick question: why is CharDeviceEntry::major a u32 while BlockDeviceEntry::major an i32 ?

[yamaura]

The difference in types is aligned with Linux kernel code. See below links.
I think it would be fine to use the same type, considering convenience for user of this library.

If you have a policy on whether to prioritize user convenience or align with the kernel regarding types, please let me know.

https://github.com/torvalds/linux/blob/master/fs/char_dev.c#L36
https://github.com/torvalds/linux/blob/master/block/genhd.c#L160

[eminence]

Thanks for those links. My general policy is to prioritize alignment with the kernel (though I think I deviate from that in a few spots). A comment in the code with these links would be useful to anyone reading this code in the future, but that can happen later.

Thanks for the pull request!