IPC: Maintain circ_supply and base_fee

[aakoshh]

As a follow up to consensus-shipyard/fendermint#14 figure out how to update the circ_supply. Are there tokens issued during FVM execution? There are burn fields, so the amount might decrease.

[adlrocha]

Related: #233, #69

As discussed sync with @guy-goren, our reference implementation will set base_fee=0 (although we will still allow subnet deployers to choose a different value).

For the circulating supply kept as part of the state in Fendermint, we will update this value with every deposit and withdrawal from/to the subnet through cross-net message, that way we will have an accurate value of the circulating supply that we can use in the FVM.

We won't be following the EIP1559 gas model (we only leverage the interface to interact with the system), which means that no funds will be burnt, and we will reward the gas fees equally to all validators (#169), except a 1% tax (parametrizable) that is sent to the governance account in the subnet.

[adlrocha]

@aakoshh, I guess we can close this one, right?. We won't be touching the base_fee in the short-term (I feel we should keep it constant until we have specific user requests).