Avoid an array allocation per element in list passed to seplist

The array allocation was because the keyword splat expression is
not recognized as safe by the compiler.  Also avoid unnecessary
>= method call per element.  This uses a private constant to
avoid unnecessary work at runtime.

I assume the only reason this code is needed is because v may
end with a ruby2_keywords hash that we do not want to treat as
keywords.

This issue was found by the performance warning in Ruby feature
21274.

Author: jeremyevans

lib/pp.rb

@@ -276,15 +276,20 @@ def comma_breakable
     def seplist(list, sep=nil, iter_method=:each) # :yield: element
       sep ||= lambda { comma_breakable }
       first = true
+      kwsplat = EMPTY_HASH
       list.__send__(iter_method) {|*v|
         if first
           first = false
         else
           sep.call
         end
-        RUBY_VERSION >= "3.0" ? yield(*v, **{}) : yield(*v)
+        kwsplat ? yield(*v, **kwsplat) : yield(*v)
       }
     end
+    EMPTY_HASH = if RUBY_VERSION >= "3.0"
+      {}.freeze
+    end
+    private_constant :EMPTY_HASH
 
     # A present standard failsafe for pretty printing any given Object
     def pp_object(obj)