Box zeroize to prevent leaving copies on move. (#358)

* Box zeroize to prevent leaving copies in memory.

* Trigger build.

README.md

@@ -19,7 +19,7 @@ First, add this to your `Cargo.toml`:
 web3 = { git = "https://github.com/tomusdrw/rust-web3" }
 ```
 
-## Examples
+## Example
 ```rust
 #[tokio::main]
 async fn main() -> web3::Result<()> {
@@ -100,7 +100,7 @@ web3.api::<CustomNamespace>().custom_method().wait().unwrap()
 # Installation on Windows
 
 Currently, Windows does not support IPC, which is enabled in the library by default.
-To complile, you need to disable IPC feature:
+To complile, you need to disable the IPC feature:
 ```
 web3 = { version = "0.11.0", default-features = false, features = ["http"] }
 ```

src/api/accounts.rs

@@ -177,7 +177,7 @@ type TxParams<T> = Join3<MaybeReady<T, U256>, MaybeReady<T, U256>, MaybeReady<T,
 /// immediately.
 pub struct SignTransactionFuture<T: Transport> {
     tx: TransactionParameters,
-    key: ZeroizeSecretKey,
+    key: Box<ZeroizeSecretKey>,
     inner: TxParams<T>,
 }
 
@@ -202,7 +202,7 @@ impl<T: Transport> SignTransactionFuture<T> {
 
         SignTransactionFuture {
             tx,
-            key: ZeroizeSecretKey(*key),
+            key: ZeroizeSecretKey::boxed(*key),
             inner,
         }
     }
@@ -350,6 +350,13 @@ impl Transaction {
 #[derive(Clone, Copy)]
 struct ZeroizeSecretKey(SecretKey);
 
+impl ZeroizeSecretKey {
+    /// Create new boxed instance to make sure we don't leak any copies around.
+    pub fn boxed(key: SecretKey) -> Box<Self> {
+        Box::new(Self(key))
+    }
+}
+
 impl Default for ZeroizeSecretKey {
     fn default() -> Self {
         ZeroizeSecretKey(ONE_KEY)