hash: provide asserts to verify API use

- converted the Curl_hash_count() macro to a function

- Discourage accessing struct fields directly

- Document the internal API in HASH.md

Closes #14503

tests/unit/unit1609.c

@@ -200,7 +200,6 @@ UNITTEST_START
 
     curl_easy_cleanup(easy);
     easy = NULL;
-    Curl_hash_destroy(&multi->hostcache);
     curl_multi_cleanup(multi);
     multi = NULL;
     curl_slist_free_all(list);