fix compilation on uefi targets

[usamoi]

• Added a CHANGELOG.md entry

Summary

On uefi targets, the return type of std::io::Error::raw_os_error is Option<usize>, instead of Option<i32>. In rand_core, it is assumed as Option<i32>. This PR uses Option<usize> as the return type on the uefi targets.

getrandom has already handled this: https://github.com/rust-random/getrandom/blob/18d89843981b93032b2a2c6f1e33897075a8d727/src/error.rs#L6-L19

[dhardy]

This doesn't feel like the right way to do this: clearly #1537 failed to isolate rand from getrandom changes.

Do you need a patch release for rand_core?

[newpavlov]

Note that technically it's a breaking change. I think we can cfg the implementation for UEFI and replace it with panic and fix it properly in the next breaking release.

UPD: On the second thought, since the current code does not compile on the UEFI targets, then this change can be considered non-breaking since it only affects "broken" targets.

[usamoi]

Do you need a patch release for rand_core?

No. I can use the patch locally, so this is not necessary for me.

Note that technically it's a breaking change.

I'm fine with waiting to merge it into 0.10.