Substantially refactor ~everything to separate concerns and improve flow clarity

Highlights:
- Remove all use of malloc and friends
- Refactor message handling functions to work with references
- Move core message read => triage => respond logic loop to main
  function
- Improve message triage and support follow-up messages
- Refactor hex byte parsing
- Disable periodic interrupt when not playing

src/queue.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <stdint.h>
+
+typedef struct Queue_struct {
+  uint8_t write;
+  uint8_t read;
+  uint8_t size;
+  void *buf[];
+} Queue_t;
+
+void constructQueue(Queue_t *q, void *buf, uint8_t size, uint8_t len,
+                    uint8_t constructFull);
+void constructEmptyQueue(Queue_t *q, uint8_t size, uint8_t len);
+uint8_t isEmpty(const Queue_t *q);
+uint8_t pushQueue(Queue_t *q, void *x);
+const void *peekQueue(const Queue_t *q);
+void *popQueue(Queue_t *q);