Add Enumerable#each_cons_pair and Iterator#cons_pair yielding a tuple

[straight-shoota]

Enumerable#each_cons has a variable chunk size that can be selected dynamically.
This PR adds a special case for the most common use case of iterating over pairs. It simply yields tuples of adjacent items. This avoids heap allocations and makes it easier to use because you don't need to deconstruct an array.

# new:
[1, 2, 3, 4, 5].each_cons do |a, b|
  puts a * b
end
# old:
[1, 2, 3, 4, 5].each_cons(2, reuse: true) do |pair|
  a, b = pair
  puts a * b
end

Benchmark:

# iterating over (1..1000).to_a
new 228.27k (  4.38µs) (±17.91%)   0.0B/op        fastest
old  33.13k ( 30.19µs) (± 9.87%)  80.0B/op   6.89× slower

It's a convenience feature that also improves performance.

I've added the same behaviour to Iterator#cons

[asterite]

I like this!

What if we name it each_cons2? Then we could eventually add each_cons3, each_cons4, etc. if we really wanted to.

Though it's true that one of the most common cases for each_cons is 2... 🤔

[straight-shoota]

I don't see a reason for adding a number suffix. Iterating over pairs is by far the most common use case. If we wanted to add others, there's nothing hindering us from adding each_cons3 later anyway.

[RX14]

Want to rebase for a merge commit?

[vlazar]

I love this optimization but I feel like the chosen size 2 mentioned explicitly in the method name like #each_cons_pair / Iterator#cons_pair would be even better.

[bcardiff]

I like @vlazar's suggestion. Otherwise, the expected block arguments differ based on the presence or not of the argument. I like the explicitness of each_cons_pair

[straight-shoota]

I guess that might be a good enhancement, but waiting for feedback on this suggestion.

[asterite]

I think each_cons_pair is good. My suggestion each_cons2, each_cons3, etc. might also be taken into consideration, though I doubt we'll spend effort adding each of these other special variants, so 👍 each_cons_pair from me.

[RX14]

+1 from me, might want to go ahead with this

[bcardiff]

each_cons_pair or each_cons2 should be used as a name. Other than that LGTM.

[straight-shoota]

Renamed to Enumerable#each_cons_pair and Iterator#cons_pair.

[bcardiff]

Merge upon renaming the PR for the changelog.