[ruby/prism] Optimize ruby visitor

`compact_child_nodes` allocates an array. We can skip that step by simply yielding the nodes.

Benchmark for visiting the rails codebase:

```rb
require "prism"
require "benchmark/ips"

files = Dir.glob("../rails/**/*.rb")
results = files.map { Prism.parse_file(it) }
visitor = Prism::Visitor.new

Benchmark.ips do |x|
  x.config(warmup: 3, time: 10)

  x.report do
    results.each do
      visitor.visit(it.value)
    end
  end
end

RubyVM::YJIT.enable

Benchmark.ips do |x|
  x.config(warmup: 3, time: 10)

  x.report do
    results.each do
      visitor.visit(it.value)
    end
  end
end
```

Before:
```
ruby 3.4.8 (2025-12-17 revision https://github.com/ruby/prism/commit/995b59f666) +PRISM [x86_64-linux]
Warming up --------------------------------------
                         1.000 i/100ms
Calculating -------------------------------------
                          2.691 (± 0.0%) i/s  (371.55 ms/i) -     27.000 in  10.089422s
ruby 3.4.8 (2025-12-17 revision https://github.com/ruby/prism/commit/995b59f666) +YJIT +PRISM [x86_64-linux]
Warming up --------------------------------------
                         1.000 i/100ms
Calculating -------------------------------------
                          7.278 (±13.7%) i/s  (137.39 ms/i) -     70.000 in  10.071568s
```
After:
```
ruby 3.4.8 (2025-12-17 revision https://github.com/ruby/prism/commit/995b59f666) +PRISM [x86_64-linux]
Warming up --------------------------------------
                         1.000 i/100ms
Calculating -------------------------------------
                          3.429 (± 0.0%) i/s  (291.65 ms/i) -     35.000 in  10.208580s
ruby 3.4.8 (2025-12-17 revision https://github.com/ruby/prism/commit/995b59f666) +YJIT +PRISM [x86_64-linux]
Warming up --------------------------------------
                         1.000 i/100ms
Calculating -------------------------------------
                         16.815 (± 0.0%) i/s   (59.47 ms/i) -    169.000 in  10.054668s
```

~21% faster on the interpreter, ~56% with YJIT

https://github.com/ruby/prism/commit/bf631750cf

3 files changed, 28 insertions, 5 deletions

diff --git a/prism/templates/lib/prism/compiler.rb.erb b/prism/templates/lib/prism/compiler.rb.erb
index 9102025c20..66dbe666b9 100644
--- a/prism/templates/lib/prism/compiler.rb.erb
+++ b/prism/templates/lib/prism/compiler.rb.erb
@@ -29,14 +29,14 @@ module Prism
 
     # Visit the child nodes of the given node.
     def visit_child_nodes(node)
-      node.compact_child_nodes.map { |node| node.accept(self) }
+      node.each_child_node.map { |node| node.accept(self) }
     end
 
     <%- nodes.each_with_index do |node, index| -%>
 <%= "\n" if index != 0 -%>
     # Compile a <%= node.name %> node
     def visit_<%= node.human %>(node)
-      node.compact_child_nodes.map { |node| node.accept(self) }
+      node.each_child_node.map { |node| node.accept(self) }
     end
     <%- end -%>
   end
diff --git a/prism/templates/lib/prism/node.rb.erb b/prism/templates/lib/prism/node.rb.erb
index ceee2b0ffe..c97c029d3b 100644
--- a/prism/templates/lib/prism/node.rb.erb
+++ b/prism/templates/lib/prism/node.rb.erb
@@ -187,7 +187,7 @@ module Prism
       while (node = queue.shift)
         result << node
 
-        node.compact_child_nodes.each do |child_node|
+        node.each_child_node do |child_node|
           child_location = child_node.location
 
           start_line = child_location.start_line
@@ -259,6 +259,13 @@ module Prism
 
     alias deconstruct child_nodes
 
+    # With a block given, yields each child node. Without a block, returns
+    # an enumerator that contains each child node. Excludes any `nil`s in
+    # the place of optional nodes that were not present.
+    def each_child_node
+      raise NoMethodError, "undefined method `each_child_node' for #{inspect}"
+    end
+
     # Returns an array of child nodes, excluding any `nil`s in the place of
     # optional nodes that were not present.
     def compact_child_nodes
@@ -335,6 +342,22 @@ module Prism
       }.compact.join(", ") %>]
     end
 
+    # def each_child_node: () { (Prism::node) -> void } -> void | () -> Enumerator[Prism::node]
+    def each_child_node
+      return to_enum(:each_child_node) unless block_given?
+
+      <%- node.fields.each do |field| -%>
+      <%- case field -%>
+      <%- when Prism::Template::NodeField -%>
+      yield <%= field.name %>
+      <%- when Prism::Template::OptionalNodeField -%>
+      yield <%= field.name %> if <%= field.name %>
+      <%- when Prism::Template::NodeListField -%>
+      <%= field.name %>.each  {|node| yield node }
+      <%- end -%>
+      <%- end -%>
+    end
+
     # def compact_child_nodes: () -> Array[Node]
     def compact_child_nodes
       <%- if node.fields.any? { |field| field.is_a?(Prism::Template::OptionalNodeField) } -%>
diff --git a/prism/templates/lib/prism/visitor.rb.erb b/prism/templates/lib/prism/visitor.rb.erb
index b1a03c3f1a..76f907724f 100644
--- a/prism/templates/lib/prism/visitor.rb.erb
+++ b/prism/templates/lib/prism/visitor.rb.erb
@@ -20,7 +20,7 @@ module Prism
     # Visits the child nodes of `node` by calling `accept` on each one.
     def visit_child_nodes(node)
       # @type self: _Visitor
-      node.compact_child_nodes.each { |node| node.accept(self) }
+      node.each_child_node { |node| node.accept(self) }
     end
   end
 
@@ -48,7 +48,7 @@ module Prism
 <%= "\n" if index != 0 -%>
     # Visit a <%= node.name %> node
     def visit_<%= node.human %>(node)
-      node.compact_child_nodes.each { |node| node.accept(self) }
+      node.each_child_node { |node| node.accept(self) }
     end
     <%- end -%>
   end