Implement Indexer for Keys in the memoria

[0xnullifier]

Have a look at indexer.go to get idea for the interface you would want to implement.

The idea of Indexer is to have some order of keys for various reasons like:

• Efficient key iteration: It is very likely you would want to iterate over the keys to do some action with them.
• Sorted Access: You basically you prevent a filesystem Walk for each key. Get keys in some sorted manner this is useful for queries say maybe you want top 10 keys by some metric.

and many more reasons. I have made issues for some common indexer check them out here:
Implement Array Sort Indexing #10
Implement BTree Indexing #11