Expand the "nullable pointer optimization" section with a code example.

Change examples to use std::os::raw instead of libc, when applicable.

Not sure the example is going to stay, but I can try to pass Travis for the bragging rights.

…ype, even though that is how it works

book/ffi: nullable pointer cleanup

Expand the "nullable pointer optimization" section with a code example. Fixes rust-lang#34250.

I also noticed that many of the examples use the libc crate just for types such as `c_char` and `c_int`, which are now available through `std::os::raw`. I changed the ones that don't need to rely on libc. I'm glad to revert that part of the commit if it's unwanted churn.

…abnik

More intuitive explantion of strings formatting

…omez

Add documentation example for `str::Chars::as_str`.

None

Fix typos

r? @steveklabnik

Update docs for assert! and debug_assert!

Refer to rust-lang#34455

Fs docs

Fixes rust-lang#29356.

r? @steveklabnik

Try to clear up some awkward wording