Automatic String casting to basic types

[mojidabckuu]

Your JSON dictionary:

{
  "id": "1"
}

Your model:

struct Repo: Mappable {
  var id: Int = 0

  init(_ map: Map) {
    id <- map["id"]
  }
}

What you did:

let repo = Mapper<Repo>().map(myJSONDictionary)

What you expected:

print(repo.id) // prints 1 not 0

What you got:

print(repo.id) // prints 0

Hey there! Thanks for the awesome tool to map the data! Above is a use case when we need to handle string values for basic types. I know, I know that it would be cool to use transforms, but it is not elegant solution to handle a such case. The proposal: Simply do next steps

1. Create a protocol

   protocol StringInitiable { 
   init?(stringValue: String) 
   }

2. Add basic extensions

   extension Int: StringInitiable {
   init?(stringValue: String) {
   guard let value = Int(stringValue) else { return nil }
   self = value
   }
   }

3. Edit operators to handle <T: StringInitiable>
4. Edit Map's value to handle a case

   if let v = currentValue as? String { return T.init(stringValue: currentValue)

Point me if I wrong but it would be awesome to automatically cast String to basic types without any effort for the end user and moreover without breaking changes.

I am happy to make the PR

Cheers, Vlad