Warn about Rust safety with a list of known pitfalls

[danielhenrymantilla]

This list should be completed with any pitfall I may have forgotten to mention, and more external references to blog posts or official documentation should be added.

Lastly, safety-related patterns vs anti-patterns (same folder organization as https://github.com/rust-unofficial/patterns) could be added.

[ad-anssi]

Hello Daniel,

Thank you for this list of ideas. Your contribution goes precisely in the
direction of this guide: warn about Rust pitfalls, either in 'unsafe', 'safe'
or 'non-unsafe' Rust.
Thus, the points you mention could probably be redistributed in already existing
sections.

I suggest you (or I, if you don't have the time to do it) post issues/PRs to start
discussions while separating the cases; for instance:

• using 'unsafe' for handling FFI is mentioned in the 'unsafe' section of the
  guide, but I agree with you on the fact that other precautions may be taken
  when doing FFI (the difficulty here is to not include too much information
  that would be specific to the foreign language),

• panic! topic has been added in 'Error handling' section, but there are still
  things to document,

• integer overflows are discussed in a dedicated section. Again, there is
  always possible improvements,

• logic bugs cannot be prevented in the general case, but some language
  constructs may be more error-prone (due to complex operations involved or
  easy misunderstanding for example). These should be documented as well.