Add nonReentrant to `relayMessage()` (#172)

* Add nonReentrant to all public functions on CrossDomainMessengers

* Improve contract description comments
Co-authored-by: ben-chain <ben@pseudonym.party>

* remove extraneous modifier
Co-authored-by: ben-chain <ben@pseudonym.party>

packages/contracts/contracts/optimistic-ethereum/OVM/bridge/OVM_BaseCrossDomainMessenger.sol

@@ -40,7 +40,7 @@ abstract contract OVM_BaseCrossDomainMessenger is iOVM_BaseCrossDomainMessenger,
         address _target,
         bytes memory _message,
         uint32 _gasLimit
-    ) nonReentrant
+    )
         override
         public
     {

packages/contracts/contracts/optimistic-ethereum/OVM/bridge/OVM_L1CrossDomainMessenger.sol

@@ -7,6 +7,7 @@ import { Lib_OVMCodec } from "../../libraries/codec/Lib_OVMCodec.sol";
 import { Lib_AddressResolver } from "../../libraries/resolver/Lib_AddressResolver.sol";
 import { Lib_AddressManager } from "../../libraries/resolver/Lib_AddressManager.sol";
 import { Lib_SecureMerkleTrie } from "../../libraries/trie/Lib_SecureMerkleTrie.sol";
+import { Lib_ReentrancyGuard } from "../../libraries/utils/Lib_ReentrancyGuard.sol";
 
 /* Interface Imports */
 import { iOVM_L1CrossDomainMessenger } from "../../iOVM/bridge/iOVM_L1CrossDomainMessenger.sol";
@@ -18,6 +19,7 @@ import { OVM_BaseCrossDomainMessenger } from "./OVM_BaseCrossDomainMessenger.sol
 
 /**
  * @title OVM_L1CrossDomainMessenger
+ * @dev This contract lives on L1. It sends L1->L2 messages into L2, and relays L2->L1 messages from L2 to their target on L1.
  */
 contract OVM_L1CrossDomainMessenger is iOVM_L1CrossDomainMessenger, OVM_BaseCrossDomainMessenger, Lib_AddressResolver {
 
@@ -81,6 +83,7 @@ contract OVM_L1CrossDomainMessenger is iOVM_L1CrossDomainMessenger, OVM_BaseCros
     )
         override
         public
+        nonReentrant
         onlyRelayer()
     {
         bytes memory xDomainCalldata = _getXDomainCalldata(

packages/contracts/contracts/optimistic-ethereum/OVM/bridge/OVM_L2CrossDomainMessenger.sol

@@ -5,6 +5,7 @@ pragma experimental ABIEncoderV2;
 
 /* Library Imports */
 import { Lib_AddressResolver } from "../../libraries/resolver/Lib_AddressResolver.sol";
+import { Lib_ReentrancyGuard } from "../../libraries/utils/Lib_ReentrancyGuard.sol";
 
 /* Interface Imports */
 import { iOVM_L2CrossDomainMessenger } from "../../iOVM/bridge/iOVM_L2CrossDomainMessenger.sol";
@@ -17,6 +18,7 @@ import { OVM_BaseCrossDomainMessenger } from "./OVM_BaseCrossDomainMessenger.sol
 /**
  * @title OVM_L2CrossDomainMessenger
  * @dev L2 CONTRACT (COMPILED)
+ * This contract lives on L2. It sends messages to L1, and relays them from L1.
  */
 contract OVM_L2CrossDomainMessenger is iOVM_L2CrossDomainMessenger, OVM_BaseCrossDomainMessenger, Lib_AddressResolver {
 
@@ -50,6 +52,7 @@ contract OVM_L2CrossDomainMessenger is iOVM_L2CrossDomainMessenger, OVM_BaseCros
         uint256 _messageNonce
     )
         override
+        nonReentrant
         public
     {
         require(