int: replace the (e)println! with log

[JiangengDong]

Currently, r2r prints error messages with println! and eprintln!. However, this is not good for debugging, because this is no call site information, and the (e)println! is hard to be silent.

In this PR, all the println! are replaced with log::debug! and all the eprintln! are replaced with log::error!. This gives the users more flexibility and better control over the output and makes it easier to locate the problem.

[JiangengDong]

Well, I accidentally also changed the println! in mod tests and in comments. I am going to push a commit to revert them later.

[m-dahl]

I feel kind of bad that you are doing all the TODOs that never got done :). This is a really needed pr 👍 Just me know when you are ready to to merge it.

[JiangengDong]

Pushed a commit to revert the println! used in tests and comments. Sorry, I have been busy last week with my regular work.

Should this be a minor version change or a patch version change?

[m-dahl]

No worries. Unless there is something more you would like to do in this PR we can just merge this for now, and I am thinking we should make a release that bumps to 0.8 once we make the change static array change.

[JiangengDong]

Rebased to the latest master branch, and pushed new commits to increase the version to 0.7.4.

I think we can merge this if there is no other concern.

[m-dahl]

I just merged this but I didn't include the version bump since I don't have time to make a release just now.