feats: dynamic price pools & router

[blackbigswan]

Relevant: #10

Both dynamic price pools and router would be most likely interconnected from protocol design perspective, therefore I put them together in one issue.

Going by what I mentioned in #10 about basket pools/multiple token pools, I think it's more interesting to choose a different direction for design of this protocol:

1. by dynamic price pools I understand what Balancer defines as WeightedPools, e.g pools of proportion different than usual 50/50 for assetA & assetB. Another example of dynamic price change pools would be liquidity mining pools. Both of those could be implemented in Helios, with Balancer WeightedMath used as base. Relevant mechanics is described here
2. router contract to allow swaps between any token pools A <> (B) <> C. an interesting solution would be to maybe use EIP-4626 for this, but I haven't yet thought this fully through if it makes sense. of course, this can be solution outsourced from what is available in protocols like uni/bal, but making it based on 4626 could potentially mean better integration with different DeFi apps in future.