Force BCF header dicts to be larger than needed.

This is a tactic to reduce hash collisions due to the use of overly
simple hash functions.  It seems to typically be around 3-8% speed gain.

vcf.c

@@ -1315,8 +1315,11 @@ bcf_hdr_t *bcf_hdr_init(const char *mode)
     bcf_hdr_t *h;
     h = (bcf_hdr_t*)calloc(1, sizeof(bcf_hdr_t));
     if (!h) return NULL;
-    for (i = 0; i < 3; ++i)
+    for (i = 0; i < 3; ++i) {
         if ((h->dict[i] = kh_init(vdict)) == NULL) goto fail;
+        // Supersize the hash to make collisions very unlikely
+        if (kh_resize(vdict, h->dict[i], 32768) < 0) goto fail;
+    }
 
     bcf_hdr_aux_t *aux = (bcf_hdr_aux_t*)calloc(1,sizeof(bcf_hdr_aux_t));
     if ( !aux ) goto fail;