standardise storage cost per byte

[frisitano]

This PR standardises the cost of storage (per byte) across the different pallets that charge for storage usage in the reef runtime. This includes transactions, evm and multisig. The cost per byte has been standardised at 10 mREEF.

closes: #62

[Netherdrake]

For EVM, some backwards compatibility testing is required (ie. will the contract terminations refund storage deposit correctly).

[frisitano]

I am currently in the process of exploring contract terminations. Contract termination via evm.selfdestruct extrinsic seems reasonable. My main concern is when a selfdestruct OPCODE is sent via an evm.call extrinsic as in this instance the destruction is handled by the EVM handler rather then the extrinsic logic. Will report back with findings.

[frisitano]

TODO:

• Add tests for self destruct event emission via evm.call SUICIDE opcode
• refactor selfdestuct method such that evm.call (SUICIDE OPCODE) + evm.selfdestruct use the same contract destruction logic (https://github.com/reef-defi/reef-chain/blob/master/modules/evm/src/lib.rs#L890-L924). Currently evm.call (SUICIDE) does not update CodeInfos and hence it could result in stale data being left behind which contributes to state bloat. Additionally evm.call uses a different storage refund mechanic.

[frisitano]

TODO:

• Investigate failure to estimate evm costs in certain situations
• End to end testing

[frisitano]

This was not ready for merge - still had bugs relating to event queue not being purged properly.