make nonce_index u16

Author: darioush

neps/nep-0611.md

@@ -209,7 +209,7 @@ As an alternative, we could use the [estimator](https://github.com/near/nearcore
 ```rust
 enum TransactionNonce {
     Nonce { nonce: Nonce },
-    GasKeyNonce { nonce: Nonce, nonce_index: u32 }
+    GasKeyNonce { nonce: Nonce, nonce_index: u16 }
 }
 ```
 
@@ -354,7 +354,7 @@ Individual nonces are stored under the following `TrieKey` as `u64` values:
     GasKeyNonce {
         account_id: AccountId,
         public_key: PublicKey,
-        nonce_index: u32
+        nonce_index: u16
     } // also uses col::ACCESS_KEY
 ```
 
@@ -439,7 +439,7 @@ As an alternative, gas keys may be stored under a separate trie column using the
     GasKeyNonce {
         account_id: AccountId,
         public_key: PublicKey,
-        nonce_index: Option<u32>
+        nonce_index: Option<u16>
     } // uses new column col::GAS_KEY
 ```