Avoid array allocations for generated entity code

Before this commit the generated entity constructor looked like:

    def initialize(attributes = {})
      @id, @name, @age, = *attributes.values_at(:id, :name, :age)
    end

After this commit the generated code remains its previous semantics:

    def initialize(attributes = {})
      @id, @name, @age = attributes.values_at(:id, :name, :age)
    end

Despite being shorter this generated code also has some performance improvements:

    require 'benchmark/ips'

    HASH = { :id => 1, :name => "foo", :age => 23 }
    id, name, age = nil

    Benchmark.ips do |x|
      x.report "simple" do
        id, name, age = HASH.values_at(:id, :name, :age)
      end
      x.report "rest" do
        id, name, age, = HASH.values_at(:id, :name, :age)
      end
      x.report "splat + rest" do
        id, name, age, = *HASH.values_at(:id, :name, :age)
      end
    end

    =begin

    Calculating -------------------------------------
                  simple    105210 i/100ms
                    rest    105217 i/100ms
            splat + rest     95173 i/100ms
    -------------------------------------------------
                  simple  3235210.9 (±0.8%) i/s -   16202340 in   5.008459s
                    rest  3224124.5 (±0.7%) i/s -   16203418 in   5.025926s
            splat + rest  2729041.1 (±0.7%) i/s -   13704912 in   5.022124s

    =end