Add NEP?: Script Nef Standard

[Tommo-L]

NEF is Neo executable format, I think we should make it a NEP standard.

namespace Neo.SmartContract
{
    /// <summary>
    /// +------------+-----------+------------------------------------------------------------+
    /// |   Field    |  Length   |                          Comment                           |
    /// +------------+-----------+------------------------------------------------------------+
    /// | Magic      | 4 bytes   | Magic header                                               |
    /// | Compiler   | 32 bytes  | Compiler used                                              |
    /// | Version    | 16 bytes  | Compiler version (Mayor, Minor, Build, Version)            |
    /// | ScriptHash | 20 bytes  | ScriptHash for the script                                  |
    /// +------------+-----------+------------------------------------------------------------+
    /// | Checksum   | 4 bytes   | Sha256 of the header (CRC)                                 |
    /// +------------+-----------+------------------------------------------------------------+
    /// | Script     | Var bytes | Var bytes for the payload                                  |
    /// +------------+-----------+------------------------------------------------------------+
    /// </summary>
    public class NefFile : ISerializable

original posted at neo-project/neo#889 neo-project/neo#903

[Tommo-L]

@bettybao1209 will help send the PR.