sha256: Use the same union order as the ctx structure

This reads better. Also remove duplicated comments for the members.
Signed-off-by: Jon Griffiths <jon_p_griffiths@yahoo.com>

ccan/crypto/sha256/sha256.h

@@ -21,10 +21,8 @@
  */
 struct sha256 {
 	union {
-		/* Array of chars */
-		unsigned char u8[32];
-		/* Array of uint32_t */
 		uint32_t u32[8];
+		unsigned char u8[32];
 	} u;
 };