Return u64 from get_balance in the athcon FFI

[poszu]

Currently, the get_balance() function returns a athcon_uint256be:

athena/ffi/athcon/include/athcon/athcon.h

Lines 549 to 559 in 96b9afa

	/**
	* Get balance callback function.
	*
	* This callback function is used by a VM to query the balance of the given account.
	*
	* @param context The pointer to the Host execution context.
	* @param address The address of the account.
	* @return The balance of the given account or 0 if the account does not exist.
	*/
	typedef athcon_uint256be (*athcon_get_balance_fn)(struct athcon_host_context *context,
	const athcon_address *address);

Which is supposed to be a 256bit big-endian integer encoded in a byte array:

athena/ffi/athcon/include/athcon/athcon.h

Lines 43 to 46 in 96b9afa

	/**
	* The alias for athcon_bytes32 to represent a big-endian 256-bit integer.
	*/
	typedef struct athcon_bytes32 athcon_uint256be;

There are two problems with it:

• the athena balance is u64 so its inefficient to pass it as 256bits
• it's supposed to be big-endian (according to the type description) but we put the value as little-endian there

IMHO, we should change the C API to return a uint64_t instead to avoid confusion, make it efficient and simplify the code.

[lrettig]

Good catch. We use u64 in go-spacemesh so we should do the same in Athena. In any case it's more general than uint256.