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

CMakeLists.txt

@@ -107,6 +107,7 @@ set(AVR_UPLOADTOOL_BASE_OPTIONS ${AVR_UPLOADTOOL_BASE_OPTIONS} -U syscfg1:w:0x4:
 add_avr_executable(mockingboard
     src/sniffer.c
     src/com232.c
+    src/queue.c
     src/avclandrv.c)
 
 target_link_options(mockingboard PUBLIC