Split current "unsafe" chapter, cover more topics

[japaric]

The current chapter needs to be split:

• intro: What does the unsafe block means? (can't be proved safe by the compiler), should be used sparingly, first suspect in case of a segfault, should be treated as a red flag by the code reviewer
• Dereferencing raw pointers
• std::mem::transmute raw pointers -> references
• asm!

We probably want to add:

• static mut

[steveklabnik]

I'd be happy to mentor anyone who wants to work on this!

[crazystylus]

Hi @steveklabnik ,
Are you still willing to mentor someone one this?
I want to give it a shot!!

[steveklabnik]

Sadly, I am not. @marioidival is the only person left working on rust by example. He may, or he may not. Good luck!

[marioidival]

Hi, I'm closing this issue just to clean up the items that already exist. If you think this issue makes sense to stay open, please create a new issue with updated information and mention this one.

thanks!