Message type - match EVMC

[mratsim]

It would probably best to match EVMC fields rather than Py-EVM to ease interface with EVM-C and eWASM in the future.

EVMC Message type - https://github.com/ethereum/evmc/blob/master/include/evmc/evmc.h

/**
 * Big-endian 256-bit integer.
 *
 * 32 bytes of data representing big-endian 256-bit integer. I.e. bytes[0] is
 * the most significant byte, bytes[31] is the least significant byte.
 * This type is used to transfer to/from the VM values interpreted by the user
 * as both 256-bit integers and 256-bit hashes.
 */
struct evmc_uint256be
{
    /** The 32 bytes of the big-endian integer or hash. */
    uint8_t bytes[32];
};

/** Big-endian 160-bit hash suitable for keeping an Ethereum address. */
struct evmc_address
{
    /** The 20 bytes of the hash. */
    uint8_t bytes[20];
};

/** The kind of call-like instruction. */
enum evmc_call_kind
{
    EVMC_CALL = 0,         /**< Request CALL. */
    EVMC_DELEGATECALL = 1, /**< Request DELEGATECALL. The value param ignored. */
    EVMC_CALLCODE = 2,     /**< Request CALLCODE. */
    EVMC_CREATE = 3        /**< Request CREATE. Semantic of some params changes. */
};

/** The flags for ::evmc_message. */
enum evmc_flags
{
    EVMC_STATIC = 1 /**< Static call mode. */
};

/**
 * The message describing an EVM call,
 * including a zero-depth calls from a transaction origin.
 */
struct evmc_message
{
    /** The destination of the message. */
    struct evmc_address destination;

    /** The sender of the message. */
    struct evmc_address sender;

    /**
     * The amount of Ether transferred with the message.
     */
    struct evmc_uint256be value;

    /**
     * The message input data.
     *
     *  This MAY be NULL.
     */
    const uint8_t* input_data;

    /**
     * The size of the message input data.
     *
     *  If input_data is NULL this MUST be 0.
     */
    size_t input_size;

    /**
     * The optional hash of the code of the destination account.
     *  The null hash MUST be used when not specified.
     */
    struct evmc_uint256be code_hash;

    /** The amount of gas for message execution. */
    int64_t gas;

    /** The call depth. */
    int32_t depth;

    /** The kind of the call. For zero-depth calls ::EVMC_CALL SHOULD be used. */
    enum evmc_call_kind kind;

    /**
     * Additional flags modifying the call execution behavior.
     *  In the current version the only valid values are ::EVMC_STATIC or 0.
     */
    uint32_t flags;
};

Current Nim type for Messages https://github.com/status-im/nimbus/blob/6f28d1186675f648e0587f44e730b9dc23291bab/nimbus/vm_types.nim#L53-L89

[mratsim]

@chfast is adding the EVMC interface to Geth in ethereum/go-ethereum#17050.

This means that we could build Geth with Nimbus backend and be much more confident about having a rock solid VM implementation.

[mratsim]

closing in favor of #114