feat(fx): add effect rack and effect units

Source/EffectUnits/Compressor.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "../FXProcessor.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Compressor — soft-knee RMS compressor with makeup gain. p[0]=threshold,
+// p[1]=ratio, p[2]=attack, p[3]=release.
+// ===========================================================================
+class CompressorUnit : public FXUnit
+{
+public:
+    void prepare (double sampleRate, int maxBlockSize) override;
+    void reset() override;
+    void process (float* l, float* r, int numSamples, const float p[4]) override;
+
+private:
+    double sr = 44100.0;
+    float  envL = 0.0f, envR = 0.0f;
+    float  gainL = 1.0f, gainR = 1.0f;
+};
+
+} // namespace serum