Protected buffer works! Vast improvements to planner efficiency. Many things still broken with overhaul.

Development push. Lots still broken.

- Protected planner concept works! This is a critical precursor to
enabling feedrate overrides in allowing the planner buffer and the
stepper execution operate atomically. This is done through a
intermediary segment buffer.

- Still lots of work to be done, as this was a complete overhaul of the
planner and stepper subsystems. The code can be cleaned up quite a bit,
re-enabling some of the broken features like feed holds, and finishing
up some of the concepts

- Pushed some of the fixes from the master and edge branch to here, as
this will likely replace the edge branch when done.

serial.c

@@ -91,11 +91,7 @@ void serial_write(uint8_t data) {
 }
 
 // Data Register Empty Interrupt handler
-#ifdef __AVR_ATmega644P__
-ISR(USART0_UDRE_vect)
-#else
-ISR(USART_UDRE_vect)
-#endif
+ISR(SERIAL_UDRE)
 {
   // Temporary tx_buffer_tail (to optimize for volatile)
   uint8_t tail = tx_buffer_tail;
@@ -144,11 +140,7 @@ uint8_t serial_read()
   }
 }
 
-#ifdef __AVR_ATmega644P__
-ISR(USART0_RX_vect)
-#else
-ISR(USART_RX_vect)
-#endif
+ISR(SERIAL_RX)
 {
   uint8_t data = UDR0;
   uint8_t next_head;