Revert "Do not cache the struct layout in the metaclass"

This reverts commit 21a8a8c.

Per ffi/ffi#734, the behavior that broke this caching is now
deprecated and will be removed in ffi 2.0.

core/src/main/java/org/jruby/ext/ffi/Struct.java

@@ -112,14 +112,8 @@ static final StructLayout getStructLayout(Ruby runtime, IRubyObject structClass)
                 throw runtime.newRuntimeError("no valid struct layout for " + ((RubyClass) structClass).getName());
             }
 
-            // FIXME: This cache breaks repeat layouts against the same class, since the first layout
-            // gets cached. It broke struct_spec.rb where it reopens PairLayout in two successive specs,
-            // but has never been reported to JRuby as a bug. This may be a case worth preventing with
-            // a hard error, to avoid the need to constantly re-retrieve the layout.
-
-//            // Cache the layout on the Struct metaclass for faster retrieval next time
-//            ((RubyClass) structClass).setFFIHandle(layout);
-
+            // Cache the layout on the Struct metaclass for faster retrieval next time
+            ((RubyClass) structClass).setFFIHandle(layout);
             return (StructLayout) layout;
 
         } catch (ClassCastException ex) {