Peak Buffer decay

[exa04]

Implements decay for the peak buffer.

Changes the peak buffer's enqueueing behavior; It now behaves like a peak meter, by immediately snapping to values greater than its previous value, but decaying in amplitude if the current value is less than the previous one.

Changes:

• Adds peek() method to the RingBuffer
• Adds decay and decay weight fields to the PeakBuffer
• Adds update() method to the PeakBuffer, for updating both the sample delta and decay weight
• The PeakBuffer's sample rate is now left uninitialized until set_sample_rate is called.

Breaking Change

The parameter list for PeakBuffer::new() has now changed:

- fn new(size: usize, sample_rate: f32, duration: f32)
+ fn new(size: usize, duration:    f32, decay:    f32)

As an example, this is how you could adapt to this new parameter list if you have a PeakBuffer that you initialize inside your plug-in's default() function.

impl Default for DemoPlugin {
    fn default() -> Self {
        Self {
-           peak_buffer: Arc::new(Mutex::new(PeakBuffer::new(800, 44100.0, 10.0)))
+           peak_buffer: Arc::new(Mutex::new(PeakBuffer::new(800, 10.0, 0.)))
        }
    }
}