Add begin_chain function to contract

[frankbraun]

For the evm-bully we need a begin_chain function.
The function should allow to set the account balances in the genesis state and should only be available in a testing version of the contract.

[artob]

@frankbraun Commit 526005d added an initial parameter definition for begin_chain:

aurora-engine/src/types.rs

Lines 44 to 48 in 78c7c1f

	/// Borsh-encoded parameters for the `begin_chain` function.
	#[derive(BorshSerialize, BorshDeserialize)]
	pub struct BeginChainArgs {
	pub chain_id: RawU256,
	}

Note that the input to the begin_chain function is Borsh-encoded, not JSON-encoded.

I'm still working on figuring out a workable Borsh encoding for setting genesis block balances.

Meanwhile, if more scalar parameters beyond chain_id are needed, add them in a PR.

[frankbraun]

This needs to be put behind a feature flag and, as @ilblackdragon noted, should only be callable by the owner.

@artob: You mentioned that you already have some code for this?

[artob]

@artob: You mentioned that you already have some code for this?

I've restored the basic begin_chain functionality in 1458997. It had broken when #6 was merged.

[joshuajbouw]

Isn't this closed now?

[artob]

@frankbraun Can this be closed? Please close issues when they are done.

[frankbraun]

Sorry, this got buried in GitHub notifications.