Implement swap fees

[jubeira]

We'll go with a straightforward approach: take a fee in the tokens, on every swap.
We'll not accrue debt between joins / exits nor pay in BPT.

The swap fee should not imbalance the pools at any time; it should just be accounted separately.

We can start by transferring the fees elsewhere and measure the gas costs; warm storage access should make this cheaper than what it was by V2 launch. If needed, we can optimize in a second iteration.