Optimize Hash#stringify_keys

Using Symbol#name allows to hit two birds with one stone.

First it will return a pre-existing string, so will save
one allocation per key.

Second, that string will be already interned, so it will
save the internal `Hash` implementation the work of looking
up the interned strings table to deduplicate the key.

```
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [x86_64-darwin21]
Warming up --------------------------------------
                to_s    17.768k i/100ms
                cond    23.703k i/100ms
Calculating -------------------------------------
                to_s    169.830k (±10.4%) i/s -    852.864k in   5.088377s
                cond    236.803k (± 7.9%) i/s -      1.185M in   5.040945s

Comparison:
                to_s:   169830.3 i/s
                cond:   236803.4 i/s - 1.39x  faster
```

```ruby
require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips', require: false
end

HASH = {
  first_name: nil,
  last_name: nil,
  country: nil,
  profession: nil,
  language: nil,
  hobby: nil,
  pet: nil,
  longer_name: nil,
  occupation: nil,
  mailing_address: nil,
}.freeze

require 'benchmark/ips'

Benchmark.ips do |x|
  x.report("to_s") { HASH.transform_keys(&:to_s) }
  x.report("cond") { HASH.transform_keys { |k| Symbol === k ? k.name : k.to_s } }
  x.compare!(order: :baseline)
end
```

activesupport/lib/active_support/core_ext/hash/keys.rb

@@ -8,13 +8,13 @@ class Hash
   #   hash.stringify_keys
   #   # => {"name"=>"Rob", "age"=>"28"}
   def stringify_keys
-    transform_keys(&:to_s)
+    transform_keys { |k| Symbol === k ? k.name : k.to_s }
   end
 
   # Destructively converts all keys to strings. Same as
   # +stringify_keys+, but modifies +self+.
   def stringify_keys!
-    transform_keys!(&:to_s)
+    transform_keys! { |k| Symbol === k ? k.name : k.to_s }
   end
 
   # Returns a new hash with all keys converted to symbols, as long as
@@ -82,14 +82,14 @@ class Hash
   #   hash.deep_stringify_keys
   #   # => {"person"=>{"name"=>"Rob", "age"=>"28"}}
   def deep_stringify_keys
-    deep_transform_keys(&:to_s)
+    deep_transform_keys { |k| Symbol === k ? k.name : k.to_s }
   end
 
   # Destructively converts all keys to strings.
   # This includes the keys from the root hash and from all
   # nested hashes and arrays.
   def deep_stringify_keys!
-    deep_transform_keys!(&:to_s)
+    deep_transform_keys! { |k| Symbol === k ? k.name : k.to_s }
   end
 
   # Returns a new hash with all keys converted to symbols, as long as

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -393,7 +393,7 @@ module ActiveSupport
 
     private
       def convert_key(key)
-        key.kind_of?(Symbol) ? key.name : key
+        Symbol === key ? key.name : key
       end
 
       def convert_value(value, conversion: nil)

activesupport/lib/active_support/json/encoding.rb

@@ -76,7 +76,13 @@ module ActiveSupport
             when Hash
               result = {}
               value.each do |k, v|
-                k = k.to_s unless String === k
+                unless String === k
+                  k = if Symbol === k
+                    k.name
+                  else
+                    k.to_s
+                  end
+                end
                 result[k] = jsonify(v)
               end
               result