Add * and -> operators to Hashmap iterator

[mikee47]

This PR enables more natural use of iteration:

HashMap<int, String> map;
map[0] = "abc";'
map[2] = "def";
for(auto entry: map) {
  Serial.println(*entry); // Previously had to use entry->value()
  Serial.println(entry->length()); // entry->value().length();
}