Add reentry protection to ExecutionManager.run() (#175)

* Add nonreentrant to ExecutionManager.run()

* Cheaper check using uninit'd tx context

* Remove extraneous import

packages/contracts/contracts/optimistic-ethereum/OVM/execution/OVM_ExecutionManager.sol

@@ -144,6 +144,7 @@ contract OVM_ExecutionManager is iOVM_ExecutionManager, Lib_AddressResolver {
         override
         public
     {
+        require(transactionContext.ovmNUMBER == 0, "Only be callable at the start of a transaction");
         // Store our OVM_StateManager instance (significantly easier than attempting to pass the
         // address around in calldata).
         ovmStateManager = iOVM_StateManager(_ovmStateManager);