Deduplicate index keys

[YaroShkvorets]

Implements #10

Reduces number of index keys 3-4x for Ethereum mainnet.

• saves substreams cache space
• improves performance (probably?)

[maoueh]

The keys are de-deduped on the backend as they are saved within a map[...]... component so it doesn't really in terms of space saving.

Now the question boils down to does adding a HashSet to dedupe keys is worth it vs the extra processing happening when we send multiple time the same key to the backing engine. My gut feeling tells me it will be more efficient to leave them as-is and avoid the HashSet.

I'm not totally sure how to prove this. I've just added a benchmarking setup on ethereum-common, I can at least probably gather the difference time this adds to see the impact of just adding HashSet.

[YaroShkvorets]

Ah yes, makes sense. Probably not worth it compacting keys in the module then. Feel free to close this PR if your benchmark doesn't show any improvement.