Improve unsafe code

[andreeaflorescu]

Summary of the PR

• Added some more comments about the unsafe code in volatile_slice.
• Added a negative test to check the behavior of read_from when the size of the read is bigger than the size of the reader.

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[andreeaflorescu]

Why can't we use #[must_use] ?

The must_use will apply to the Result, and Result is already must_use in Rust. I couldn't find any way to specify that the inner type in a Result is a must_use. The only alternative that I could think of is changing the return type to a custom result type, but that comes with a breaking change and I didn't find it to be worth it. Do you know about an alternative to this? I might be missing something.