feat(fx): add effect rack and effect units

Source/EffectUnits/Phaser.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "../FXProcessor.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// Phaser — cascade of four first-order allpass stages with an LFO-modulated
+// coefficient and feedback. p[0]=rate, p[1]=depth, p[2]=feedback, p[3]=stages.
+// ===========================================================================
+class PhaserUnit : 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;
+    double lfoPhase = 0.0;
+    float  xL[6] {}, xR[6] {}, yL[6] {}, yR[6] {};
+};
+
+} // namespace serum