CString's `as_ptr` is easy to get very wrong

[spaolacci]

Whilst the documentation explicitly mentions the situation, precise scope is easy to get wrong.

// .as_ptr returns an internal pointer into the CString, and may be invalidated when the                                                                                        
// CString falls out of scope (the destructor will run, freeing the allocation if there is                                                                                      
// one).                                                                                                                                                                        
let s = "foo";                                                                                                                                                                  
let s1 = s.to_c_str(); // Ok.                                                                                                                                          
let s2 = s.to_c_str().as_ptr(); // Oops.                                                                                                                                        
//         ^~~~ tmp CString falls out of scope.                                                                                                                                 

let (c1, c2) = unsafe{ (*s1.as_ptr(), *s2) };                                                                                                                                   
println!("{}, {}", c1, c2); //  Prints `102, 0'

The same pattern is even easier to overlook in e.g libc::open(filename.to_c_str().as_ptr(), ...).

String's with_c_str should probably be preferred (or documented as preferred).

[aturon]

Dupe of #16035