Add a contains() method to silicondb::map

[arindas]

Add a new method to silicondb::map :

// map::contains(): returns true if the map contains a value mapped to the given key, false otherwise.
template <typename Key, typename Value, typename Hash = std::hash<Key>>
bool silicondb::map::contains(Key const& key) { ... }

The implementation will require adding a contains() method for map::bucket. We simply need to check whether, in the bucket mapped to the hash, there is an entry with the given key or not. (Hint: use find_entry_for)