Inline functions and make parameters const

Apparently the Arduino compiler ignores "inline" because it tries to
optimise for size. However, all functions are only called once so
there shouldn't be any size penalty for inlining.

src/main.cpp

@@ -10,9 +10,9 @@ const uint_fast8_t PIN_MUTE = 9;
 const byte MIDI_ALL_SOUNDS_OFF = 120;
 const byte MIDI_ALL_NOTES_OFF = 123;
 
-bool is_reset_msg(const MidiMessage& msg);
-void set_note(uint_fast8_t note, uint_fast8_t octave);
-void set_voltage(float voltage);
+inline bool is_reset_msg(const MidiMessage& msg) __attribute__((always_inline));
+inline void set_note(uint_fast8_t note, uint_fast8_t octave) __attribute__((always_inline));
+inline void set_voltage(float voltage) __attribute__((always_inline));
 
 void setup()
 {
@@ -52,12 +52,12 @@ bool is_reset_msg(const MidiMessage& msg)
         && (msg.param1 == MIDI_ALL_NOTES_OFF || msg.param1 == MIDI_ALL_SOUNDS_OFF);
 }
 
-void set_note(uint_fast8_t note, uint_fast8_t octave)
+void set_note(const uint_fast8_t note, const uint_fast8_t octave)
 {
     set_voltage(octave + (note / 12.0F));
 }
 
-void set_voltage(float voltage)
+void set_voltage(const float voltage)
 {
     PORTD = static_cast<uint8_t>(lroundf(ONE_VOLT * voltage));
 }