Rework Params trait API with Arc instead of Pin

This is a breaking change requiring a small change to plugin
implementations.

The reason why `Pin<&dyn Params>` was used was more as a hint to
indicate that the object must last for the plugin's lifetime, but `Pin`
doesn't enforce that. It also makes the APIs a lot more awkward.
Requiring the use of `Arc` fixes the following problems:

- When storing the params object in the wrapper, the `ParamPtr`s are
  guaranteed to be stable.
- This makes it possible to access the `Params` object without acquiring
  a lock on the plugin, this is very important for implementing
  plugin-side preset management.
- It enforces immutability on the `Params` object.
- And of course the API is much nicer without a bunch of unsafe code to
  work around Pin's limitations.

nih_plug_iced/src/lib.rs

@@ -12,14 +12,14 @@
 //! }
 //!
 //! pub(crate) fn create(
-//!     params: Pin<Arc<FooParams>>,
+//!     params: Arc<FooParams>,
 //!     editor_state: Arc<IcedState>,
 //! ) -> Option<Box<dyn Editor>> {
 //!     create_iced_editor::<Foo>(editor_state, params)
 //! }
 //!
 //! struct FooEditor {
-//!     params: Pin<Arc<FooParams>>,
+//!     params: Arc<FooParams>,
 //!     context: Arc<dyn GuiContext>,
 //!
 //!     foo_slider_state: nih_widgets::param_slider::State,
@@ -34,7 +34,7 @@
 //! impl IcedEditor for FooEditor {
 //!     type Executor = executor::Default;
 //!     type Message = Message;
-//!     type InitializationFlags = Pin<Arc<FooParams>>;
+//!     type InitializationFlags = Arc<FooParams>;
 //!
 //!     fn new(
 //!         params: Self::InitializationFlags,
@@ -141,7 +141,7 @@ pub fn create_iced_editor<E: IcedEditor>(
 
 /// A plugin editor using `iced`. This wraps around [`Application`] with the only change being that
 /// the usual `new()` function now additionally takes a `Arc<dyn GuiContext>` that the editor can
-/// store to interact with the parameters. The editor should have a `Pin<Arc<impl Params>>` as part
+/// store to interact with the parameters. The editor should have a `Arc<impl Params>` as part
 /// of their [`InitializationFlags`][Self::InitializationFlags] so it can read the current parameter
 /// values. See [`Application`] for more information.
 pub trait IcedEditor: 'static + Send + Sync + Sized {

nih_plug_iced/src/widgets/generic_ui.rs

@@ -5,7 +5,7 @@ use atomic_refcell::AtomicRefCell;
 use std::borrow::Borrow;
 use std::collections::HashMap;
 use std::marker::PhantomData;
-use std::pin::Pin;
+use std::sync::Arc;
 
 use nih_plug::prelude::{Param, ParamFlags, ParamPtr, Params};
 
@@ -58,7 +58,7 @@ pub struct GenericSlider;
 pub struct GenericUi<'a, W: ParamWidget> {
     state: &'a mut State<W>,
 
-    params: Pin<&'a dyn Params>,
+    params: Arc<dyn Params>,
 
     width: Length,
     height: Length,
@@ -85,7 +85,7 @@ where
     W: ParamWidget,
 {
     /// Creates a new [`GenericUi`] for all provided parameters.
-    pub fn new(state: &'a mut State<W>, params: Pin<&'a dyn Params>) -> Self {
+    pub fn new(state: &'a mut State<W>, params: Arc<dyn Params>) -> Self {
         Self {
             state,