💥 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/gain_gui_egui/src/lib.rs

@@ -120,7 +120,7 @@ impl Plugin for Gain {
                                     setter.end_set_parameter(&params.gain);
                                     new_value
                                 }
-                                None => params.gain.value as f64,
+                                None => params.gain.value() as f64,
                             }
                         })
                         .suffix(" dB"),