gcode_parse.h: order variables reasonably in groups of 4 bytes.

This uses 4 bytes less RAM, without any loss, due to fewer holes
in variable arrangements.

The general strategy is simple:

 - Ideally, all variables are aligned in groups of 4 bytes
   (32 bits). This allows fastest access on 32-bit CPUs and doesn't
   change anything on 8 or 16 bit ones.

 - 1x 32-bits variable = 4 bytes = 4-byte group.

 - 2x 16-bits variable together = 4-byte group.

 - 4x 8-bits variable together = 4-byte group.

 - Have as few incomplete groups as possible.

Another strategy is to simply order variables by size.

There's a compiler flag to pack such variable arrangements, but
this costs Flash size and processing time.

gcode_parse.h

@@ -45,18 +45,18 @@ typedef struct {
 		uint8_t					option_inches				:1; ///< inches or millimeters?
 	};
 
+  uint32_t          N;          ///< line number
+  uint32_t          N_expected; ///< expected line number
+
+  int32_t           S;          ///< S word (various uses)
+  uint16_t          P;          ///< P word (various uses)
+
 	uint8_t						G;				///< G command number
 	uint8_t						M;				///< M command number
 	TARGET						target;		///< target position: X, Y, Z, E and F
 
-	int32_t						S;				///< S word (various uses)
-	uint16_t					P;				///< P word (various uses)
-
 	uint8_t						T;				///< T word (tool index)
 
-	uint32_t					N;				///< line number
-	uint32_t					N_expected;	///< expected line number
-
 	uint8_t						checksum_read;				///< checksum in gcode command
 	uint8_t						checksum_calculated;	///< checksum we calculated
 } GCODE_COMMAND;