fix: mark PIDFDInfo and PIDRUsage unsafe

[ethanpailes]

This patch marks the PIDRUsage and PIDFDInfo traits as unsafe, since incorrect implementations of these traits could lead to unsound behavior.

Unfortunately, since these are public traits, this will mean an incompatible version bump for the next release, though hopefully no one is really rolling their own implementation for these guys.

https://doc.rust-lang.org/reference/unsafe-keyword.html#unsafe-traits-unsafe-trait

[andrewdavidmackenzie]

Clippy fails due to missing docs for UNSAFE

[andrewdavidmackenzie]

Fixes #146 I understand?

[ethanpailes]

The test failures should be fixed now. Sorry it took a minute, I had notifications routing to the wrong email.