💥 Use interior mutability for parameters

Instead of the previous technically-unsound approach. While it wouldn't
cause any issues in practice, it did break Rust's guarantees. That was a
design choice after adding support for editors in NIH-plug, but this is
probably the better long term solution.

The downside is that all uses of `param.value` now need to be changed to
`param.value()`.

plugins/examples/sine/src/lib.rs

@@ -150,7 +150,7 @@ impl Plugin for Sine {
             let gain = self.params.gain.smoothed.next();
 
             // This plugin can be either triggered by MIDI or controleld by a parameter
-            let sine = if self.params.use_midi.value {
+            let sine = if self.params.use_midi.value() {
                 // Act on the next MIDI event
                 while let Some(event) = next_event {
                     if event.timing() > sample_id as u32 {