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_egui/src/widgets/generic_ui.rs

@@ -1,7 +1,7 @@
 //! A simple generic UI widget that renders all parameters in a [`Params`] object as a scrollable
 //! list of sliders and labels.
 
-use std::pin::Pin;
+use std::sync::Arc;
 
 use egui::{TextStyle, Ui, Vec2};
 use nih_plug::prelude::{Param, ParamFlags, ParamPtr, ParamSetter, Params};
@@ -35,7 +35,7 @@ pub struct GenericSlider;
 /// space.
 pub fn create(
     ui: &mut Ui,
-    params: Pin<&dyn Params>,
+    params: Arc<impl Params>,
     setter: &ParamSetter,
     widget: impl ParamWidget,
 ) {