Improve binary documentation

[JayKickliter]

For my colleagues and I, one of the hardest parts of coming up to speed with Rustler has been dealing with binaries. Even those of us with years of Rust experience were confused by the difference between Binary and OwnedBinary, despite the fact that Owned should say it all. I've tried my best to capture my confusion before I'm too familiar with codebase.

Included in this PR:

• add module-level for the binary module, thus I had to remove the doc_hidden tag
• When adding a # Panics section for OwnedBinary::from_unowned, I noticed that it could be simplified to the point that it's obvious that it will not panic (it calls copy_from_slice internally, which can panic, but won't since lengths will be checked before that call)
• documents all binary api functions, even if trivially, for completeness sake

For reviewers: please run cargo doc --open and try all the intra-doc links in the binary module if possible. I've rustdoc to be very unreliable at warning on broken links.

[JayKickliter]

@filmor thanks for all the feedback, I'll use all your suggestions. I figured I had factual errors, hence opening an early draft PR.

[JayKickliter]

I think I have addressed most/all of @filmor's comments, but may have introduced more factual errors in the process.

I've gotten a little busy lately, so I may not be able complete everything in the in checklist in the PR overview at the top, but I'll leave this PR as draft for little a longer.

[JayKickliter]

I removed the draft tag as this PR is about as complete as I can make it. I've made made some changes since @filmor's initial review of the draft version, so I welcome any further comments.

[evnu]

Very nice, thanks! I only found two minor nits.