Custom serializers

[robacarp]

This replaces the attempt at a general purpose (de)serializer by case statement with an extensible, customize-able, method based approach.

sort of fixes #16

This opens the door for a wider variety of orms and other object storage mechanisms to easily implement serialization, which will make fixing issues like #15 #14 much easier.

Custom serializers (or serializer overrides) are implemented by defining a pair of methods on a job called serialize_<type>. For example, the methods which handle Int32 are:

def serialize_int32(value : Int32) : String
  value.to_s
end

def deserialize_int32(raw : String) : Int32?
  if raw
    raw.to_i32
  end
end

These methods are declared in the abstract Job class, which means that they're override-able too, to your hearts content:

class PutsJob < Mosquito::QueuedJob
   params(count : Int32 | Nil)

  def perform
    count.times { puts "puts job" }
  end

  def deserialize_int32(raw : String) : Int32
    puts "custom deserialization, with a default value!"
    if raw
      raw.to_i32
    else
      0
    end
  end
end

Serializing ORM models gets a little more complicated, but I think this will be a better abstraction. To match the current functionality, I'm including a Serializers::Granite module, which provides a macro to generate serializers for model classes. It's used like this:

class PutsJob < Mosquito::QueuedJob
  include Mosquito::Serializers::Granite

  params(user : User | Nil)

  serialize_granite_model User

  def perform
    puts "Hello user #{user.id}"
  end
end

serialize_granite_model(User) generates these methods:

def serialize_user(model : User) : String
  model.id.to_s
end

def deserialize_user(raw : String) : User
  id = raw.to_i
  User.find id
end