Make Hash#each family usable in Ractor

[kou]

We don't need to increment/decrement iteration level for frozen Hash
because frozen Hash can't be modified. We can assume that nobody
changes the target Hash while calling #each family.

How to reproduce:

a = {}
100.times do |i|
  a[i] = true
end
Ractor.make_shareable(a)

4.times.collect do
  Ractor.new(a) do |b|
    100.times do
      b.each_value do
      end
    end
  end
end.each(&:take)

Example output:

internal:ractor>:267: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
#<Thread:0x00007fcfb087bb30 run> terminated with exception (report_on_exception is true):
#<Thread:0x00007fcfb087b8d8 run> terminated with exception (report_on_exception is true):
#<Thread:0x00007fcfb088d678 run> terminated with exception (report_on_exception is true):
#<Thread:0x00007fcfb087bd88 run> terminated with exception (report_on_exception is true):
/tmp/h.rb:10:in `each_value'/tmp/h.rb:10:in `each_value': : /tmp/h.rb:10:in `each_value'no implicit conversion from nil to integer/tmp/h.rb:10:in `each_value'no implicit conversion from nil to integer (: :  (TypeErrorTypeError)no implicit conversion from nil to integer)no implicit conversion from nil to integer (

 (TypeErrorTypeError	from /tmp/h.rb:10:in `block (3 levels) in <main>'
	from /tmp/h.rb:10:in `block (3 levels) in <main>'
))	from /tmp/h.rb:9:in `times'

	from /tmp/h.rb:9:in `times'

	from /tmp/h.rb:9:in `block (2 levels) in <main>'
	from /tmp/h.rb:10:in `block (3 levels) in <main>'
	from /tmp/h.rb:10:in `block (3 levels) in <main>'
	from /tmp/h.rb:9:in `block (2 levels) in <main>'
	from /tmp/h.rb:9:in `times'
	from /tmp/h.rb:9:in `times'
	from /tmp/h.rb:9:in `block (2 levels) in <main>'
	from /tmp/h.rb:9:in `block (2 levels) in <main>'
<internal:ractor>:694:in `take': thrown by remote Ractor. (Ractor::RemoteError)
	from /tmp/h.rb:14:in `each'
	from /tmp/h.rb:14:in `<main>'
/tmp/h.rb:10:in `each_value': no implicit conversion from nil to integer (TypeError)
	from /tmp/h.rb:10:in `block (3 levels) in <main>'
	from /tmp/h.rb:9:in `times'
	from /tmp/h.rb:9:in `block (2 levels) in <main>'

[kou]

@ko1 @nobu What do you think about this change?

[nobu]

This could move into the else block below.

[kou]

Ah, right.
I'll do.

[kou]

Done.

[kou]

CI is green.
I'll merge this.