Performance improvements, calculate attributes to serialize only once

[Bhacaz]

1.1704x faster

Benchmark

require_relative './lib/bright_serializer'

class User
  attr_accessor :friends, :id, :first_name, :last_name
  def initialize(id, first_name, last_name)
    @id = id
    @first_name = first_name
    @last_name = last_name
  end
end

class UserSerializer
  include BrightSerializer::Serializer
  attributes :id, :first_name, :last_name

  attribute :friends do |object|
    UserSerializer.new(object.friends, fields: [:id, :first_name])
  end
end


list = 100.times.map { |i| User.new(i, "first_name_#{i}", "last_name_#{i}") }
list.each do |user|
  user.friends = list.sample(10).map { |u| nu = u.dup; nu.friends = nil; nu }
end

require 'benchmark/ips'

Benchmark.ips do |x|
  x.report('A') do
    UserSerializer.new(list).to_hash
  end
  x.compare!
end

# A      1.104k (± 1.4%) i/s -      5.618k in   5.089321s
# A    959.739  (± 1.6%) i/s -      4.800k in   5.002612s
# 1.1704x faster

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Fix and add specs for coverage