enumerator.c: Introduce Enumerator::ArithmeticSequence

This commit introduces new core class Enumerator::ArithmeticSequence.
Enumerator::ArithmeticSequence is a subclass of Enumerator, and
represents a number generator of an arithmetic sequence.

After this commit, Numeric#step and Range#step without blocks
returned an ArithmeticSequence object instead of an Enumerator.

This class introduces the following incompatibilities:

- You can create a zero-step ArithmeticSequence,
  and its size is not ArgumentError, but Infinity.
- You can create a negative-step ArithmeticSequence from a range.

[ruby-core:82816] [Feature #13904]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64205 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_enumerator.rb

@@ -632,7 +632,7 @@ class TestEnumerator < Test::Unit::TestCase
     assert_equal 4, (1..10).step(3).size
     assert_equal 3, (1...10).step(3).size
     assert_equal Float::INFINITY, (42..Float::INFINITY).step(2).size
-    assert_raise(ArgumentError){ (1..10).step(-2).size }
+    assert_equal 0, (1..10).step(-2).size
   end
 
   def test_size_for_downup_to