USB Improvements

* Introduce shell module for basic serial shell with argument parsing
* Introduce shell_cmd_list module for basic compile-time command
  registration
* Harden USB handling to hang less and drop fewer inputs
  - Service tud_task() with periodic TC0 timer interrupt
  - Service cdc_task() with periodic TC1 timer interrupt
  - Handle shell servicing in main app loop
  - Add a circular buffering layer for reads/writes
* Change newline prints to also send carriage return
* Refactor filesystem commands for shell subsystem
* Introduce new shell commands:
  - 'help' command
  - 'flash' command to reset into bootloader
  - 'stress' command to stress CDC writes

Testing:
* Shell validated on Sensor Watch Blue w/ Linux host
* Shell validated in emscripten emulator
* Tuned by spamming inputs during `stress` cmd until stack didn't crash

movement/make/Makefile

@@ -49,6 +49,8 @@ SRCS += \
   ../../littlefs/lfs_util.c \
   ../movement.c \
   ../filesystem.c \
+  ../shell.c \
+  ../shell_cmd_list.c \
   ../watch_faces/clock/simple_clock_face.c \
   ../watch_faces/clock/world_clock_face.c \
   ../watch_faces/clock/beats_face.c \