Add io.ReaderAt interface compatibility

[qrpike]

Read is now a proxy for ReadAt. Read uses the returned bytes count to set the new offset for the file, since ReadAt does not alter the offset.

[eikenb]

I still think this might be possible to merge in.. I just have had zero time to test it out. I'll try to get to it as soon as I can find the time. Thanks.

[eikenb]

Sorry for the long delay getting to this. It is still on my radar and I'm trying to make more time for this project. I hope to get to it soon. Thank you for your patience.

[puellanivis]

This one might be good to get in for a 1.12 release.

[nightlyone]

@puellanivis / @eikenb This looks like the last pull request that needs to be merged for a v1.12.0 release.

Would you mind doing that merge and then the release as well? I see no further work blocking the release.

[puellanivis]

So, I’m going to merge this, but also immediately start a new branch to add a mutex to protect the Read() call from multiple parallel calls.

It will be a lot safer than someone trying to figure out the inevitable race condition.