Feature/encode struct names : using boost.type index to disambiguate variants instead of fields collection

[daminetreg]

Hi @linkineo,

Could you help me reviewing this ?
It is in regards of disambiguating variants with the struct name.

This ends up in such json when a member struct is in a variant, this however drops variant disambiguation for std::string vs double vs int, because I don't disambiguate anymore for them.

Additionally it has the advantage that I don't rely on trying the deserialization process to ensure it is the right type.

[
  {
    "name": "King",
    "responsibility": {
      "checkout_number": 1,
      "section": "hardware",
      "struct": "datamodel::cashier"
    }
  },
  {
    "name": "Blake",
    "responsibility": {
      "fighting_tactic": "Krav Maga",
      "has_a_weapon": true,
      "struct": "datamodel::security"
    }
  },
  {
    "name": "Martin",
    "responsibility": {
      "floor": "5th floor",
      "room": "Toys, Petshop, Drugs, Food",
      "struct": "datamodel::cleaner"
    }
  },
  {
    "name": "Ward",
    "responsibility": {
      "checkout_number": 2,
      "section": "Food",
      "struct": "datamodel::cashier"
    }
  }
]