Make SecureRandom support Ractor

SecureRandom lazily defines `get_random`.  Accessing the mutex to define
the `get_random` method is not supported inside a Ractor.  This commit
defines `gen_random` when `securerandom` is required and makes it
suppore Ractor (as well as thread safe).

Here is a test program:

```ruby
require "securerandom"

r = Ractor.new do
  loop do
    Ractor.yield SecureRandom.hex
  end
end

p r.take
```

Before this commit:

```
$ make runruby
./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb --extout=.ext  -- --disable-gems ./test.rb
<internal:ractor>:38: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
/Users/aaron/git/ruby/lib/securerandom.rb:94:in `gen_random': can not access instance variables of classes/modules from non-main Ractors (RuntimeError)
<internal:ractor>:124:in `take': thrown by remote Ractor. (Ractor::RemoteError)
	from ./test.rb:9:in `<main>'
/Users/aaron/git/ruby/lib/securerandom.rb:94:in `gen_random': can not access instance variables of classes/modules from non-main Ractors (RuntimeError)
	from /Users/aaron/git/ruby/lib/securerandom.rb:155:in `random_bytes'
	from /Users/aaron/git/ruby/lib/securerandom.rb:176:in `hex'
	from ./test.rb:5:in `block (2 levels) in <main>'
	from ./test.rb:4:in `loop'
	from ./test.rb:4:in `block in <main>'
make: *** [runruby] Error
```

After this commit:

```
$ make runruby
./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb --extout=.ext  -- --disable-gems ./test.rb
<internal:ractor>:38: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
"3fc8885157e3911bab4b5d7619bb0308"
```

lib/securerandom.rb

@@ -66,42 +66,11 @@
 #
 
 module SecureRandom
-  @rng_chooser = Mutex.new # :nodoc:
-
   class << self
     def bytes(n)
       return gen_random(n)
     end
 
-    def gen_random(n)
-      ret = Random.urandom(1)
-      if ret.nil?
-        begin
-          require 'openssl'
-        rescue NoMethodError
-          raise NotImplementedError, "No random device"
-        else
-          @rng_chooser.synchronize do
-            class << self
-              remove_method :gen_random
-              alias gen_random gen_random_openssl
-              public :gen_random
-            end
-          end
-          return gen_random(n)
-        end
-      else
-        @rng_chooser.synchronize do
-          class << self
-            remove_method :gen_random
-            alias gen_random gen_random_urandom
-            public :gen_random
-          end
-        end
-        return gen_random(n)
-      end
-    end
-
     private
 
     def gen_random_openssl(n)
@@ -129,6 +98,21 @@ module SecureRandom
       end
       ret
     end
+
+    ret = Random.urandom(1)
+    if ret.nil?
+      begin
+        require 'openssl'
+      rescue NoMethodError
+        raise NotImplementedError, "No random device"
+      else
+        alias gen_random gen_random_openssl
+      end
+    else
+      alias gen_random gen_random_urandom
+    end
+
+    public :gen_random
   end
 end