Support for std::bitset

[finger563]

Added support for (de-)serialization of std::bitset using alpaca.

Related to #21 (since std::bitfield provides some of the same use case as struct bitfields).

Example showing bitfield serialization and deserialization works:

Test code:

    fmt::print("Starting bitfield serialization example!\n");
    //! [bitfield serialization example]
    struct MyBits {
      std::vector<bool> bits; // testing to see if std::vector<bool> space-efficient bitfield
                              // serialization works
      std::bitset<4> bitset;  // testing to see if std::bitset space-efficient bitfield
                              // serialization works
    };
    std::error_code ec;
    // here we try to make 4 bits followed by 4 bits
    MyBits object{{true, false, true, false}, 0b1010};
    fmt::print("object.bits:   {}\n", object.bits);
    fmt::print("object.bitset: {}\n", object.bitset);
    std::vector<uint8_t> buffer;
    auto bytes_written = alpaca::serialize<alpaca::options::none>(object, buffer);
    fmt::print("Serialized {}B to std::vector\n", bytes_written);
    fmt::print("Serialized buffer: {::x}\n", buffer);
    auto new_object = alpaca::deserialize<alpaca::options::none, MyBits>(buffer, ec);
    if (!ec && new_object.bits == object.bits && new_object.bitset == object.bitset) {
      fmt::print("Deserialized successfully!\n");
      fmt::print("\tbits:   {}\n"
                 "\tbitset: {}\n",
                  new_object.bits, new_object.bitset);
    } else {
      fmt::print("Deserialization failed: {}\n", ec.message());
      fmt::print("\tbits:   {}\n"
                 "\tbitset: {}\n",
                  new_object.bits, new_object.bitset);
    }
    //! [bitfield serialization example]