Add new specs. Added Enumerator.produce to generate an Enumerator from any custom data transformation
user
```ruby
require_relative '../../spec_helper'

ruby_version_is "2.7" do
  describe "Enumerator.produce" do
    it "creates an infinite enumerator" do
      enum = Enumerator.produce(0) { |prev| prev + 1 }
      enum.take(5).should == [0, 1, 2, 3, 4]
    end

    it "terminates iteration when block raises StopIteration exception" do
      enum = Enumerator.produce(0) do | prev|
        raise StopIteration if prev >= 2
        prev + 1
      end

      enum.to_a.should == [0, 1, 2]
    end

    context "when initial value skipped" do
      it "uses nil instead" do
        ScratchPad.record []
        enum = Enumerator.produce { |prev| ScratchPad << prev; (prev || 0) + 1 }

        enum.take(3).should == [1, 2, 3]
        ScratchPad.recorded.should == [nil, 1, 2]
      end

      it "starts enumerable from result of first block call" do
        io = StringIO.new("a\nb\nc\nd")
        lines = Enumerator.produce { io.gets }.take_while { |s| s }

        lines.should == ["a\n", "b\n", "c\n", "d"]
      end
    end
  end
end
```
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