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.

plugins/examples/stft/src/lib.rs

@@ -2,14 +2,13 @@ use nih_plug::prelude::*;
 use realfft::num_complex::Complex32;
 use realfft::{ComplexToReal, RealFftPlanner, RealToComplex};
 use std::f32;
-use std::pin::Pin;
 use std::sync::Arc;
 
 const WINDOW_SIZE: usize = 2048;
 const OVERLAP_TIMES: usize = 4;
 
 struct Stft {
-    params: Pin<Box<StftParams>>,
+    params: Arc<StftParams>,
 
     /// An adapter that performs most of the overlap-add algorithm for us.
     stft: util::StftHelper,
@@ -56,7 +55,7 @@ impl Default for Stft {
             .unwrap();
 
         Self {
-            params: Box::pin(StftParams::default()),
+            params: Arc::new(StftParams::default()),
 
             stft: util::StftHelper::new(2, WINDOW_SIZE),
             window_function: util::window::hann(WINDOW_SIZE),
@@ -91,8 +90,8 @@ impl Plugin for Stft {
     const ACCEPTS_MIDI: bool = false;
     const SAMPLE_ACCURATE_AUTOMATION: bool = true;
 
-    fn params(&self) -> Pin<&dyn Params> {
-        self.params.as_ref()
+    fn params(&self) -> Arc<dyn Params> {
+        self.params.clone()
     }
 
     fn accepts_bus_config(&self, config: &BusConfig) -> bool {