feat(library/init/meta/vm): add decidable_eq for vm_obj_kind

library/init/meta/vm.lean

@@ -5,6 +5,7 @@ Authors: Leonardo de Moura
 -/
 prelude
 import init.meta.tactic init.data.option.basic
+import init.meta.mk_dec_eq_instance
 
 meta constant vm_obj : Type
 
@@ -14,6 +15,9 @@ inductive vm_obj_kind
 | environment | tactic_state | format
 | options | other
 
+instance vm_obj_kind_dec_eq : decidable_eq vm_obj_kind :=
+    by tactic.mk_dec_eq_instance
+
 namespace vm_obj
 meta constant kind            : vm_obj → vm_obj_kind
 /- For simple and constructor vm_obj's, it returns the constructor tag/index.