bitmap: use external iterator

Reuse the Iterator object from hashmap.h and expose a similar API.

This allows us to do

{
    Iterator i;
    unsigned n;

    BITMAP_FOREACH(n, b, i) {
        Iterator j;
        unsigned m;

        BITMAP_FOREACH(m, b, j) {
            ...
        }
    }
}

without getting confused. Requested by David.

src/resolve/resolved-dns-packet.c

@@ -536,6 +536,7 @@ fail:
 }
 
 static int dns_packet_append_types(DnsPacket *p, Bitmap *types, size_t *start) {
+        Iterator i;
         uint8_t window = 0;
         uint8_t len = 0;
         uint8_t bitmaps[32] = {};
@@ -548,7 +549,7 @@ static int dns_packet_append_types(DnsPacket *p, Bitmap *types, size_t *start) {
 
         saved_size = p->size;
 
-        BITMAP_FOREACH(n, types) {
+        BITMAP_FOREACH(n, types, i) {
                 uint8_t entry;
 
                 assert(n <= 0xffff);

src/resolve/resolved-dns-rr.c

@@ -527,10 +527,11 @@ static int format_timestamp_dns(char *buf, size_t l, time_t sec) {
 static char *format_types(Bitmap *types) {
         _cleanup_strv_free_ char **strv = NULL;
         _cleanup_free_ char *str = NULL;
+        Iterator i;
         unsigned type;
         int r;
 
-        BITMAP_FOREACH(type, types) {
+        BITMAP_FOREACH(type, types, i) {
                 if (dns_type_to_string(type)) {
                         r = strv_extend(&strv, strdup(dns_type_to_string(type)));
                         if (r < 0)