Generate unrolled list of entity attributes

This commit improves the performance of Entity's constructor
by unrolling the list of its attributes.

Before the generated code looked like:

    def initialize(attributes = {})
      @a, @b, @c, = *attributes.values_at(:a, :b, :c)
    end

After this commit it looks like:

    def initialize(attributes = {})
      @a = attributes[:a]
      @b = attributes[:b]
      @c = attributes[:c]
    end

The benchmark:

    require 'benchmark/ips'
    require 'lotus/entity'

    def attributes_for(times)
      times.times.map { |i| :"field#{i}" }
    end

    def hash_for(times)
      Hash[times.times.map { |i| [:"field#{i}", i] }]
    end

    class Model1
      include Lotus::Entity
      self.attributes = attributes_for(2)
    end

    class Model5
      include Lotus::Entity
      self.attributes = attributes_for(5)
    end

    class Model10
      include Lotus::Entity
      self.attributes = attributes_for(10)
    end

    h1 = hash_for(2)
    h5 = hash_for(5)
    h10 = hash_for(10)

    Benchmark.ips do |x|
      x.report "model1" do
        Model1.new(h1)
      end
      x.report "model5" do
        Model5.new(h5)
      end
      x.report "model10" do
        Model10.new(h10)
      end
    end

    =begin

    ## initial

    Calculating -------------------------------------
                  model1     75310 i/100ms
                  model5     51012 i/100ms
                 model10     40219 i/100ms
    -------------------------------------------------
                  model1  1551154.1 (±2.1%) i/s -    7756930 in   5.003060s
                  model5   812667.3 (±1.4%) i/s -    4080960 in   5.022625s
                 model10   580076.2 (±1.4%) i/s -    2935987 in   5.062472s

    ## w/o splat

    Calculating -------------------------------------
                  model1     80699 i/100ms
                  model5     53507 i/100ms
                 model10     44218 i/100ms
    -------------------------------------------------
                  model1  1760277.4 (±1.8%) i/s -    8876890 in   5.044533s
                  model5   915549.1 (±1.7%) i/s -    4601602 in   5.027518s
                 model10   672009.3 (±1.7%) i/s -    3360568 in   5.002209s

    ## loop unrolled

    Calculating -------------------------------------
                  model1     86138 i/100ms
                  model5     61557 i/100ms
                 model10     47912 i/100ms
    -------------------------------------------------
                  model1  2110472.0 (±2.1%) i/s -   10594974 in   5.022513s
                  model5  1186649.0 (±1.0%) i/s -    5971029 in   5.032338s
                 model10   747101.7 (±0.9%) i/s -    3737136 in   5.002571s

    =end