Swap tid and pos in bam1_core_t to eliminate struct holes

Removes four bytes of padding between tid and pos, and four bytes
between mtid and mpos.

Author: daviesrob

htslib/sam.h

@@ -200,8 +200,8 @@ extern const int8_t bam_cigar_table[256];
  @field  mpos    0-based leftmost coordinate of next read in template
  */
 typedef struct {
-    int32_t tid;
     hts_pos_t pos;
+    int32_t tid;
     uint16_t bin; // NB: invalid on 64-bit pos
     uint8_t qual;
     uint8_t l_extranul;

test/sam.c

@@ -985,7 +985,7 @@ static void samrecord_layout(void)
     size_t bam1_t_size, bam1_t_size2;
 
     assert(sizeof(hts_pos_t) == 8 || sizeof(hts_pos_t) == 4);
-    int core_size = sizeof(hts_pos_t) == 8 ? 56 : 36;
+    int core_size = sizeof(hts_pos_t) == 8 ? 48 : 36;
 
     bam1_t_size = core_size + sizeof (int) + 4 + sizeof (char *);
 #ifndef BAM_NO_ID
@@ -994,7 +994,8 @@ static void samrecord_layout(void)
     bam1_t_size2 = bam1_t_size + 4;  // Account for padding on some platforms
 
     if (sizeof (bam1_core_t) != core_size)
-        fail("sizeof bam1_core_t is %zu, expected 56", sizeof (bam1_core_t));
+        fail("sizeof bam1_core_t is %zu, expected %d",
+             sizeof (bam1_core_t), core_size);
 
     if (sizeof (bam1_t) != bam1_t_size && sizeof (bam1_t) != bam1_t_size2)
         fail("sizeof bam1_t is %zu, expected either %zu or %zu",