Add description of memory safety.

[kbeyls]

Also discuss how memory allocators can mitigate against temporal memory safety.

[g-kouv]

We should describe what memory safety means in general, e.g. by describing a taxonomy like here:
https://security.apple.com/blog/towards-the-next-generation-of-xnu-memory-safety#memorysafety

We can also mention more formal approaches to memory safety, e.g. the notion of noninterference, described in "The Meaning of Memory Safety" by Arthur Azevedo de Amorim, Catalin Hritcu, Benjamin C. Pierce (https://arxiv.org/pdf/1705.07354).