Make OpenSSL::Digest subclasses Ractor-safe

Replace define_method with class_eval string eval for initialize,
digest, and hexdigest on the dynamically-created Digest subclasses
(SHA1, SHA256, etc.). The lambda passed to define_method captures
a local variable, making it non-shareable and preventing the method
from being called in non-main Ractors.

Author: paracycle

ext/openssl/lib/openssl/digest.rb

@@ -27,16 +27,19 @@ def self.digest(name, data)
     end
 
     %w(MD4 MD5 RIPEMD160 SHA1 SHA224 SHA256 SHA384 SHA512).each do |name|
-      klass = Class.new(self) {
-        define_method(:initialize, ->(data = nil) {super(name, data)})
-      }
+      klass = Class.new(self)
+      # Use class_eval with a string so the initialize method is not
+      # defined via define_method with a Proc (which is not Ractor-safe).
+      klass.class_eval <<-RUBY, __FILE__, __LINE__ + 1
+        def initialize(data = nil)
+          super("#{name}", data)
+        end
+      RUBY
 
-      singleton = (class << klass; self; end)
-
-      singleton.class_eval{
-        define_method(:digest) {|data| new.digest(data)}
-        define_method(:hexdigest) {|data| new.hexdigest(data)}
-      }
+      klass.singleton_class.class_eval <<-RUBY, __FILE__, __LINE__ + 1
+        def digest(data) = new.digest(data)
+        def hexdigest(data) = new.hexdigest(data)
+      RUBY
 
       const_set(name.tr('-', '_'), klass)
     end