rust: Fix deprecated use of panic!(format!(...))

warning: panic message is not a string literal
 --> src/bin/event_leak.rs:9:30
  |
9 |             Err(e) => panic!(format!(" could not create handle {:?}", e)),
  |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(non_fmt_panic)]` on by default
  = note: this is no longer accepted in Rust 2021
  = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

generator/bindtests.ml

@@ -992,7 +992,7 @@ and generate_rust_bindtests () =
   pr "fn main() {\n";
   pr "    let g = match Handle::create() {\n";
   pr "        Ok(g) => g,\n";
-  pr "        Err(e) => panic!(format!(\" could not create handle {:?}\", e)),\n";
+  pr "        Err(e) => panic!(\"could not create handle {:?}\", e),\n";
   pr "    };\n";
   generate_lang_bindtests (
     fun f args optargs ->

rust/src/bin/event.rs

@@ -5,7 +5,7 @@ fn main() {
     for _ in 0..256 {
         let mut g = match Handle::create() {
             Ok(g) => g,
-            Err(e) => panic!(format!(" could not create handle {:?}", e)),
+            Err(e) => panic!("could not create handle {:?}", e),
         };
         g.set_event_callback(
             |e, _, _, _| match e {

rust/src/bin/event_leak.rs

@@ -6,7 +6,7 @@ fn main() {
     for _ in 0..256 {
         let mut g = match Handle::create() {
             Ok(g) => g,
-            Err(e) => panic!(format!(" could not create handle {:?}", e)),
+            Err(e) => panic!("could not create handle {:?}", e),
         };
         g.set_event_callback(
             |e, _, _, _| match e {