Lookup table for calculating amount commitments
## What

When retrieving pre-RCT outputs from the DB we need to calculate an amount commitment for use in RCT rings, this is currently done manually:

https://github.com/Cuprate/cuprate/blob/27767690ca7d7c5b44172a857581f08c728a740d/storage/blockchain/src/ops/output.rs#L158-L160

Also when adding V2 miner txs:

https://github.com/Cuprate/cuprate/blob/27767690ca7d7c5b44172a857581f08c728a740d/storage/blockchain/src/ops/tx.rs#L123-L129

This issue is for creating a lookup table for common amounts (the validly decomposed amounts).

## Why

Calculating commitments is slower than using a look up table.

## How

I did this in our test sync branch:

https://github.com/Cuprate/cuprate/blob/1eaf32bee956b66591f1537201c16969d5ec8696/helper/src/commitment.rs#L55-L62

However performance tests should be conducted to see if the slow path (line 60) is actually faster than our current method 
