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/dc_token.rs

@@ -42,7 +42,7 @@ pub fn dc_token_lookup(
             params![namespc as i32, foreign_id as i32],
             0,
         )
-        .map(|s| unsafe { to_cstring(s) })
+        .map(|s| unsafe { s.strdup() })
         .unwrap_or_else(|| std::ptr::null_mut())
 }