FactoryVecDeque crash after clear and push

[euclio]

If you modify the factory_advanced example like so:

diff --git a/relm4-examples/examples/factory_advanced.rs b/relm4-examples/examples/factory_advanced.rs
index 0dbae91..287b471 100644
--- a/relm4-examples/examples/factory_advanced.rs
+++ b/relm4-examples/examples/factory_advanced.rs
@@ -41,7 +41,10 @@ impl AppUpdate for AppModel {
                 });
             }
             AppMsg::RemoveLast => {
-                self.counters.pop_back();
+                self.counters.clear();
+                self.counters.push_front(Counter {
+                    value: 0,
+                });
             }
             AppMsg::CountAt(weak_index) => {
                 if let Some(index) = weak_index.upgrade() {

Then click on "add" twice and then "remove", the application will crash with the following stack trace:

thread 'main' panicked at 'index out of bounds: the len is 2 but the index is 2', /home/euclio/repos/relm4/src/factory/collections/factory_vec_deque.rs:331:13
stack backtrace:
   0: rust_begin_unwind
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/panicking.rs:517:5
   1: core::panicking::panic_fmt
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/panicking.rs:100:14
   2: core::panicking::panic_bounds_check
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/panicking.rs:76:5
   3: <usize as core::slice::index::SliceIndex<[T]>>::index_mut
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/slice/index.rs:190:14
   4: core::slice::index::<impl core::ops::index::IndexMut<I> for [T]>::index_mut
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/slice/index.rs:26:9
   5: <alloc::vec::Vec<T,A> as core::ops::index::IndexMut<I>>::index_mut
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/alloc/src/vec/mod.rs:2508:9
   6: relm4::factory::collections::factory_vec_deque::FactoryVecDeque<Data>::compile_changes
             at /home/euclio/repos/relm4/src/factory/collections/factory_vec_deque.rs:331:13
   7: <relm4::factory::collections::factory_vec_deque::FactoryVecDeque<Data> as relm4::factory::Factory<Data,View>>::generate
             at /home/euclio/repos/relm4/src/factory/collections/factory_vec_deque.rs:347:26
   8: <factory_advanced::AppWidgets as relm4::traits::Widgets<factory_advanced::AppModel,()>>::view
             at ./examples/factory_advanced.rs:216:9
   9: relm4::app::RelmApp<Model>::with_app::{{closure}}
             at /home/euclio/repos/relm4/src/app.rs:83:17
  10: glib::main_context_channel::dispatch
             at /home/euclio/.cargo/registry/src/github.com-1ecc6299db9ec823/glib-0.14.8/src/main_context_channel.rs:242:20
  11: g_main_context_dispatch
  12: <unknown>
  13: g_main_context_iteration
  14: g_application_run
  15: <O as gio::application::ApplicationExtManual>::run_with_args
             at /home/euclio/.cargo/registry/src/github.com-1ecc6299db9ec823/gio-0.14.8/src/application.rs:30:13
  16: <O as gio::application::ApplicationExtManual>::run
             at /home/euclio/.cargo/registry/src/github.com-1ecc6299db9ec823/gio-0.14.8/src/application.rs:23:9
  17: relm4::app::RelmApp<Model>::run
             at /home/euclio/repos/relm4/src/app.rs:37:9
  18: factory_advanced::main
             at ./examples/factory_advanced.rs:231:5
  19: core::ops::function::FnOnce::call_once
             at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Expected behavior: the app does not crash. All existing counters are removed and a new counter is added.

[AaronErhardt]

Should be fixed with 7933cd6. The fix should even yield a small performance improvement.

[euclio]

Thanks for the quick fix!