move Integer#downto to Ruby

Speeds up ChunkyPNG.

The interpreter is about 70% faster:

```
before: ruby 3.4.0dev (2024-07-03T15:16:17Z master 786cf9db48) [arm64-darwin23]
after: ruby 3.4.0dev (2024-07-03T15:32:25Z ruby-downto 0b8b744ce2) [arm64-darwin23]

----------  -----------  ----------  ----------  ----------  -------------  ------------
bench       before (ms)  stddev (%)  after (ms)  stddev (%)  after 1st itr  before/after
chunky-png  892.2        0.1         526.3       1.0         1.65           1.70
----------  -----------  ----------  ----------  ----------  -------------  ------------
Legend:
- after 1st itr: ratio of before/after time for the first benchmarking iteration.
- before/after: ratio of before/after time. Higher is better for after. Above 1 represents a speedup.
```

YJIT is 2.5x faster:

```
before: ruby 3.4.0dev (2024-07-03T15:16:17Z master 786cf9db48) +YJIT [arm64-darwin23]
after: ruby 3.4.0dev (2024-07-03T15:32:25Z ruby-downto 0b8b744ce2) +YJIT [arm64-darwin23]

----------  -----------  ----------  ----------  ----------  -------------  ------------
bench       before (ms)  stddev (%)  after (ms)  stddev (%)  after 1st itr  before/after
chunky-png  709.4        0.1         278.8       0.3         2.35           2.54
----------  -----------  ----------  ----------  ----------  -------------  ------------
Legend:
- after 1st itr: ratio of before/after time for the first benchmarking iteration.
- before/after: ratio of before/after time. Higher is better for after. Above 1 represents a speedup.
```

1 files changed, 29 insertions, 0 deletions

diff --git a/numeric.rb b/numeric.rb
index 4dc406fd23..ebe06a7666 100644
--- a/numeric.rb
+++ b/numeric.rb
@@ -241,6 +241,35 @@ class Integer
     self
   end
 
+  # call-seq:
+  #   downto(limit) {|i| ... } -> self
+  #   downto(limit)            ->  enumerator
+  #
+  # Calls the given block with each integer value from +self+ down to +limit+;
+  # returns +self+:
+  #
+  #   a = []
+  #   10.downto(5) {|i| a << i }              # => 10
+  #   a                                       # => [10, 9, 8, 7, 6, 5]
+  #   a = []
+  #   0.downto(-5) {|i| a << i }              # => 0
+  #   a                                       # => [0, -1, -2, -3, -4, -5]
+  #   4.downto(5) {|i| fail 'Cannot happen' } # => 4
+  #
+  # With no block given, returns an Enumerator.
+  def downto to
+    Primitive.attr! :inline_block
+    unless defined?(yield)
+      return Primitive.cexpr! 'SIZED_ENUMERATOR(self, 1, &to, int_downto_size)'
+    end
+
+    from = self
+    while from >= to
+      yield from
+      from = from.pred
+    end
+  end
+
   #  call-seq:
   #    to_i -> self
   #