Runtime: Avoid panics in apply_extrinsic

[gavofyork]

NOTE: This is specific to the Polkadot/Demo implementations, NOT Substrate.

The native runtime should be able to just the validity of including an extrinsic upon only the information in the extrinsic and basic balance/index information of the sender account. This makes the implementation of the tx queue more straightforward and efficient and also helps make clear arguments against DoS vectors.

Basically, the only conditions upon which apply_extrinsic may panic are:

• the free balance of the sender account is less than cost_xt_basic + cost_xt_byte * xt.encode().len(); or
• the index of the sender account is not equal to xt.index.

If these "panic" conditions are not met then apply_extrinsic must never panic. To panic thereafter would cause a DoS vector for the miner at best, and will cause the miner to create invalid blocks at worst.

Directly it is determined that apply_extrinsic will not panic, the balance should be reduced by the fee and the sender index incremented. Any further "higher-level" criteria that are not met (and would thus cause a panic in the current code) should be reworked to ensure they return instead without changing any storage items (except, of course, the balance reduction and index increment).