Use unordered maps for VST3 plugin instances

The better algorithmic time complexity should help when using many (say,
hundreds) of instances of a single VST3 plugin.

src/plugin/bridges/vst3-impls/plugin-proxy.h

@@ -529,7 +529,7 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
         /**
          * Memoizes `IEditController::getParameterInfo()`.
          */
-        std::map<int32, Steinberg::Vst::ParameterInfo> parameter_info;
+        std::unordered_map<int32, Steinberg::Vst::ParameterInfo> parameter_info;
     };
 
     /**

src/plugin/bridges/vst3.h

@@ -204,7 +204,7 @@ class Vst3PluginBridge : PluginBridge<Vst3Sockets<std::jthread>> {
      * `register_plugin_proxy()` in the constractor, and an instance is then
      * removed through a call to `unregister_plugin_proxy()` in the destructor.
      */
-    std::map<size_t, std::reference_wrapper<Vst3PluginProxyImpl>>
+    std::unordered_map<size_t, std::reference_wrapper<Vst3PluginProxyImpl>>
         plugin_proxies;
 
    private: