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_dehtml.rs

@@ -53,7 +53,7 @@ pub unsafe fn dc_dehtml(buf_terminated: *mut libc::c_char) -> *mut libc::c_char
     dc_saxparser_parse(&mut saxparser, buf_terminated);
     free(dehtml.last_href as *mut libc::c_void);
 
-    to_cstring(dehtml.strbuilder)
+    dehtml.strbuilder.strdup()
 }
 
 unsafe fn dehtml_text_cb(