Remove to_cstring() naming convention ambiguity

Add a trait for str.strdup() to replace to_cstring() which avoid the
signature ambiguity with .to_string().

Also instruduce CString::yolo() as a shortcut to
CString::new().unwrap() and use it whenever the variable does can be
deallocated by going out of scope.  This is less error prone.

Use some Path.to_c_string() functions where possible.

src/context.rs

@@ -483,7 +483,7 @@ pub unsafe fn dc_get_info(context: &Context) -> *mut libc::c_char {
         fingerprint_str,
     );
 
-    to_cstring(res)
+    res.strdup()
 }
 
 pub unsafe fn dc_get_version_str() -> *mut libc::c_char {